Make commits to Git remotes

[jeremydw]

Jetway needs the ability to write files back to Git remotes. If I add a Gerrit-hosted (or GitHub hosted, etc.) git remote to Jetway, Jetway needs to be able to commit to those remotes. Initially, this capability will be used so that we can build a PO file editor for translations. Eventually, we will use this to build a lightweight frontmatter/content editing system for content authors.