Make credits configurable on Stanford CME

* Assumes cert_data stanza now has additional "CREDITS" subdict, with a
  key "COUNT" and a key "TYPE". These are interpolated directly onto the
  certificate.

@stvstnfrd, please. @caesar2164, fyi.

My hope, @sarina, is not only that this will help us, but that it will make the cme template marginally more useful to members of the open source community.... (But firstly it helps us by letting us support courses worth lots of different credit values.)