Added post-review client capability checking and git moved-files support.

Tested the following cases:
1. Posted a review with up-to-date RB and rbtools. Moved files were tracked.
2. Posted a review where the RB server has moved_files=False. Moved files were not tracked.
3. Posted a review where the RB server has no "compatabilities". Moved files were not tracked.
4. Posted a review where the RB server has no "moved_files" capability (and again with no "diffs" category). Moved files were not tracked.

Unit tests pass, don't really know how to write tests for this since it requires a server (mock objects?).