Init c.incrementalRecolorFlag in Commands.ctor.  This prevents failed unit tests when running them externally.

Leo build: 20140922055627
The NullColorizer is now a subclass of ColorizerMixin.  This correctly defines scanColorDirectives, etc, which are needed when running unit tests externally.

Leo build: 20140922055851
Improved docstrings and traces in testing code.
The docstring now makes clear that Leo's core does not use  leoBridgeTest.py.
The leoBridge documentation should reference leoBridgeTest.py.

Leo build: 20140922060108
1. Disabled some unit tests that fail when run externally. These tests are not significant, although they could be made to work if various Null classes were more capable.  Low priority.
2. Use leo.core. leoQt to import Qt-related modules.

Leo build: 20140922060345