/request_cert AJAX endpoint

Certificates: URL endpoint for cert gen, setting to enable rolling certification, and sample YAML for 'grade me' button.
    
* API endpoint for certificate generation, an authenticated  post with course
      id requests that grading be carried out and a cert generated for
      request.user in that course, using the usual grading and certificate
      machinery (ie, it does not imply whitelisting, though whitelists and
      blacklists will be respected)
    - Logs each request as it comes in
    - Calls xq.add_cert() and consequently, does grading synchronously on
        this app host and then queues request for certificate agent.
    - example usage:
    ```
        curl --data "student_id=9999&course_id=Stanford/2013/Some_Class" http://127.0.0.1:8000/request_certificate
    ```
    
* Studio advanced setting added, "certificates_show_before_end", which
      determines whether a course should permit certificates to be downloadable
      by students before the coures's end date has passed.
   - Modifications to dashboard view and templates to allow display of
        certificate download links before course has ended.
        (XXX: may declare failing students as failing before the course has ended.)
   - To test, turn the setting on in a course which hasn't ended yet, and
        force certificate generation for a student, then check their
        dashboard.

* YAML for inserting sample HTML and javascript to show how you might integrate the
      /request_certificate endpoint into the courseware.
    
* TODO:
   - make grading calls into a celery task so they happen asynchronously (?)
   - needs tests
   - needs pep8/pylint