Improve our capabilities support to not hard-code 2 level resolution.

Our capabilities are no longer limited to a category.name format.
Capabilities.has_capability can take variable arguments and traverse the
capabilities list from that. For example, passing 'foo', 'bar', 'foobar'
will properly navigate {'foo': {'bar': {'foobar': True}}}.
Change the capability for Perforce moved files support.

This updates the capability check to use the new
scmtools.perforce.moved_files capability going into RB 1.7.3. This is
used instead of diffs.moved_files so that we don't generate a new-style
Perforce diff with moved file info for older versions of Review Board
that can't support it.
Release RBTools 0.4.3.
Merge branch 'api'