Local Execution

This pull request allows a celery working to pull work from the queue and execute it without dispatching to any other back-end services.

The execution is done through python's subprocess module, it is therefore both platform independent and difficult to lock down resources (for safety).

This is enabled by putting:
    COMPUTATION_SUBMISSION_URL = 'local'
in your local.py.

The same set of tests pass/fail as other pull requests.