Vagrant, Fabric, Travis, oh my.

OK, this is a monster pull request that includes several overlapping devops integrations:

* Vagrant: See install.md for an explanation of how this works. Basically this gives us a uniform cross-platform virtual dev environment with a one-line install.
* Fabric: Tool for administrative and deployment tasks. In the django project dir, try `fab -l`. Useful commands include `fab south_out`, `fab south_in`, `fab init_test_db`, and `fab prod deploy`.
* Travis: automatic testing of every push and every pull request. Someone with a @harvard-lil admin account will have to set this up at https://travis-ci.org/ , but it's pretty straightforward.