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)
* 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.
* This version is aware of Stanford's CME Registration hack, which
  creates a professional_designation field, used to carry titles such as
  PhD or Msc. This now gets passed in to the certification call
* 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
Minimal refactoring for rolling certificates

* 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.
* TODO:
  - needs tests
  - needs pep8/pylint
Adding 'Grade Me' YAML file
Certificates: Additional flags for regenerate_user

* -G allows explicit setting of grade condition (e.g., a string such as
  'Distinction', 'Pass', 'F', 'B') for the certificate, regardless of
  grader output for the user.
* -T allows explicit request of a particular certificate template to be
  applied to a given user's certificate regeneration, i.e.:
    -T doctor_distinction.pdf
  to generate a certificate of distinction for doctors.