Capabilities with better moved files support.

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

Git diffs will no longer show renames/moves/copies by default. If the
Review Board server supports moves, then git diff will use -M to show
move information.

Reviewed at http://reviews.reviewboard.org/r/2918/
Fix the location where we load server capabilities.

The load_capabilities call was being called too soon, before the server
was invoked, due to some other code shuffling.
Make sure unicode URLs are encoded as UTF-8

If the server URL is unicode, then trying to make requests would end up with an
encoding conflict between different headers. The bug included a simple fix that
makes urllib happy: manually encoding the url as utf-8 into a string object.

Fixes bug 2238.