Return nil in methods that return a pointer.

Xcode 6 won’t convert NO to a pointer.
Update to recommend project settings.

For Xcode 6.
Ignore sentest deprecation warnings.

Required to build in Xcode 6.
Include Foundation in GTObject.h
Remove unused header from GTFilter.
Define modules
Remove non-module include from GTFilterSource.h

Forward declare git_filter_source.