Improve thread handling

[mdickinson]

Importing mapping.enable.api starts a long-running thread, and there's no API provided for shutting that thread down.

This should be fixed by:

• Only starting the thread when it's needed, and not at import time.
• Providing a way to shut the thread down cleanly.

Python 2.7.10 |Master 2.1.0.dev1829 (64-bit)| (default, Oct 21 2015, 09:09:19) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import threading
>>> threading._active
{140735095477008: <_MainThread(MainThread, started 140735095477008)>}
>>> import mapping.enable.api
2016-12-05 15:06:24.732 Python[39157:d07] CoreText performance note: Client called CTFontCreateWithName() using name "Lucida Grande" and got font with PostScript name "LucidaGrande". For best performance, only use PostScript names when calling this API.
2016-12-05 15:06:24.732 Python[39157:d07] CoreText performance note: Set a breakpoint on CTFontLogSuboptimalRequest to debug.
>>> threading._active
{140735095477008: <_MainThread(MainThread, started 140735095477008)>, 4727361536: <RequestingThread(Thread-1, started daemon 4727361536)>}