Make credits configurable on Stanford CME

* Assumes cert_data stanza now has additional "CREDITS" key, with a
  sentence fragment describing the number and type of credits available
  for the course.

* Adds documentation to the CME renderer explaining the various
  cert-data configuration parameters which it respects. Adds
  documentation to the README explaining that internationalization
  should be handled by forking.
Merge pull request #34 from edx/jrbl/stanford_cme_configurable_creds

Make credits configurable on Stanford CME