WIP: Standardize build tools using make

* Add a `Makefile` for running top-level commands:
    - `make install` installs all dependencies (assuming Ubuntu 12.04)
    - `make test` runs the test suite (both JS and Python)
    - `make` both installs and runs the tests.
* Deprecate custom Vagrantfile in favor of using devstack directly.
* Deprecate some scripts that we don't need anymore (celery workers, benchmark AI, etc.)
* Update the README
* Use PhantomJS instead of Firefox so we don't need to install xvfb to run the tests headless.

Some of the file upload JS tests failed under PhantomJS.  The tests were passing a URL of `undefined` to `$.ajax`, which under Firefox is a no-op but is an error under PhantomJS.  To address this, I refactored the tests to stub out the Ajax call entirely (using dependency injection of an `OpenAssessment.FileUploader` object).  I'll need help getting a sandbox setup with CORS so I can make sure this doesn't break file upload.

@jarv @stephensanchez