Adding git-p4 support

This was taken from a previous patch request
(http://reviews.review-board.org/r/889/) and modified for the current 0.5
release. Git-p4 only supports pushing to perforce from the master branch but
the rbtools modifications allow sending reviews from and git branch.

original github pull request: https://github.com/reviewboard/rbtools/pull/16

Testing done:
The patch has been thoroughly tested within our organisation from version 0.2
right through to 0.4.1 and with the latest 0.5 development branch. We have
roughly 20 users using git-p4 on a daily basis.

Reviewed at https://reviews.reviewboard.org/r/3122/