URL endpoint for requesting certificate generation

* API endpoint for certificate generation, a post with student id and
  course id requests that a cert be generated for that student for that
  course, using the usual grading and certificate machinery (ie, it does
  not imply whitelisting, though whitelists and blacklists will be
  respected)
* 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
  ```
* TODO:
  - make grading calls into a celery task so they happen asynchronously
  - needs tests
  - needs pep8/pylint