This patch adds support for Gerrit as a hosting service when the Gerrit

server has the gerrit-reviewboard plugin installed (see: /r/8919/ et

al.). The Gerrit API does not provide sufficient information (e.g., it

cannot retrieve files by blob ID, cannot list commits, and cannot

retrieve diffs for arbitrary commits not related to changes), so we rely

on the plugin for this. This hosting service will check for the plugin's

presence via the Gerrit API and will not allow repositories without it

installed.

Tested post-commit review request creation.
Tested rbt post with a Gerrit-backed repository.
Ran unit tests.