[`dealer`](https://github.com/klen/dealer) is a new library that I'm adding to the project in this pull request: it exists to determine the currently git commit hash, and expose that value to the templates. Adding this context processor is [the documented way to do that](https://github.com/klen/dealer#django-support), but it only works for Django templates, not for Mako templates -- which is why I still need to modify the mitxmako module to add the revision to the template context. Nonetheless, I wanted to include this context processor for consistency.