Add stub verify method.

I was going to implement gnupg, but the python bindings for it are GPL
licensed. I need to find an appropriately-licensed alternative.

This verify method does no actual verification.

Related #4

golddust/packages.py

@@ -14,6 +14,7 @@
 
 
 import json
+import warnings
 
 
 """GoldDust Packages Classes/Utilities
@@ -65,6 +66,15 @@ def from_package_file(cls, file):
         # TODO: Dependencies
         return package
 
+    def verify(self):
+        """Verify the integrity of this package.
+
+        This verifies the package tarball using its detached PGP signature.
+        """
+        warnings.warn("Package verification is not yet implemented. "
+                     "You are working with an unverified package.")
+        return True
+
 
 class InstallScript:
     """Package pre/post install action script.