[ReactNative] Make it easier to generate offline bundles

Summary:
This helps generating offline bundles by adding almost empty main.jsbundle to
the app. The only thing left is to fetch minified bundle from server and save it
to the file, then flip the switch in AppDelegate.

I'll add docs about this soon.

facebook#240

Test Plan:
Did the `curl` command from comments. Changed URL to be local file,
started SampleApp, works fine without server.

Examples/SampleApp/SampleApp.xcodeproj/project.pbxproj

@@ -9,6 +9,7 @@
 /* Begin PBXBuildFile section */
 		00481BE81AC0C86700671115 /* libRCTWebSocketDebugger.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 00481BE61AC0C7FA00671115 /* libRCTWebSocketDebugger.a */; };
 		00481BEA1AC0C89D00671115 /* libicucore.dylib in Frameworks */ = {isa = PBXBuildFile; fileRef = 00481BE91AC0C89D00671115 /* libicucore.dylib */; };
+		008F07F31AC5B25A0029DE68 /* main.jsbundle in Resources */ = {isa = PBXBuildFile; fileRef = 008F07F21AC5B25A0029DE68 /* main.jsbundle */; };
 		00C302E51ABCBA2D00DB3ED1 /* libRCTActionSheet.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 00C302AC1ABCB8CE00DB3ED1 /* libRCTActionSheet.a */; };
 		00C302E61ABCBA2D00DB3ED1 /* libRCTAdSupport.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 00C302B41ABCB8E700DB3ED1 /* libRCTAdSupport.a */; };
 		00C302E71ABCBA2D00DB3ED1 /* libRCTGeolocation.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 00C302BA1ABCB90400DB3ED1 /* libRCTGeolocation.a */; };
@@ -92,6 +93,7 @@
 /* Begin PBXFileReference section */
 		00481BDB1AC0C7FA00671115 /* RCTWebSocketDebugger.xcodeproj */ = {isa = PBXFileReference; lastKnownFileType = "wrapper.pb-project"; name = RCTWebSocketDebugger.xcodeproj; path = ../../Libraries/RCTWebSocketDebugger/RCTWebSocketDebugger.xcodeproj; sourceTree = "<group>"; };
 		00481BE91AC0C89D00671115 /* libicucore.dylib */ = {isa = PBXFileReference; lastKnownFileType = "compiled.mach-o.dylib"; name = libicucore.dylib; path = usr/lib/libicucore.dylib; sourceTree = SDKROOT; };
+		008F07F21AC5B25A0029DE68 /* main.jsbundle */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; name = main.jsbundle; path = iOS/main.jsbundle; sourceTree = "<group>"; };
 		00C302A71ABCB8CE00DB3ED1 /* RCTActionSheet.xcodeproj */ = {isa = PBXFileReference; lastKnownFileType = "wrapper.pb-project"; name = RCTActionSheet.xcodeproj; path = ../../Libraries/ActionSheetIOS/RCTActionSheet.xcodeproj; sourceTree = "<group>"; };
 		00C302AF1ABCB8E700DB3ED1 /* RCTAdSupport.xcodeproj */ = {isa = PBXFileReference; lastKnownFileType = "wrapper.pb-project"; name = RCTAdSupport.xcodeproj; path = ../../Libraries/AdSupport/RCTAdSupport.xcodeproj; sourceTree = "<group>"; };
 		00C302B51ABCB90400DB3ED1 /* RCTGeolocation.xcodeproj */ = {isa = PBXFileReference; lastKnownFileType = "wrapper.pb-project"; name = RCTGeolocation.xcodeproj; path = ../../Libraries/Geolocation/RCTGeolocation.xcodeproj; sourceTree = "<group>"; };
@@ -189,6 +191,7 @@
 		13B07FAE1A68108700A75B9A /* SampleApp */ = {
 			isa = PBXGroup;
 			children = (
+				008F07F21AC5B25A0029DE68 /* main.jsbundle */,
 				13B07FAF1A68108700A75B9A /* AppDelegate.h */,
 				13B07FB01A68108700A75B9A /* AppDelegate.m */,
 				13B07FB51A68108700A75B9A /* Images.xcassets */,
@@ -405,6 +408,7 @@
 			isa = PBXResourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				008F07F31AC5B25A0029DE68 /* main.jsbundle in Resources */,
 				13B07FBF1A68108700A75B9A /* Images.xcassets in Resources */,
 				13B07FBD1A68108700A75B9A /* LaunchScreen.xib in Resources */,
 			);

Examples/SampleApp/iOS/AppDelegate.m

@@ -31,7 +31,7 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   // OPTION 2
   // Load from pre-bundled file on disk. To re-generate the static bundle, run
   //
-  // $ curl http://localhost:8081/Examples/SampleApp/index.ios.bundle -o main.jsbundle
+  // $ curl 'http://localhost:8081/Examples/SampleApp/index.ios.bundle?dev=false&minify=true' -o iOS/main.jsbundle
   //
   // and uncomment the next following line
   // jsCodeLocation = [[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];

Examples/SampleApp/iOS/main.jsbundle

@@ -0,0 +1,5 @@
+// Offline JS
+// To re-generate the offline bundle, run this from root of your project
+// $ curl 'http://localhost:8081/Examples/SampleApp/index.ios.bundle?dev=false&minify=true' -o iOS/main.jsbundle
+
+throw new Error('Offline JS file is empty. See iOS/main.jsbundle for instructions');