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.)
Then it may not be possible to internationalize this template renderer, since it's built around the concatenation of sentence fragments sort of intrinsically. To a greater or lesser extent, they all are. 

The essential idea is moving away from hardcoded credit values; I was hoping that it would make i18n easier but if it doesn't, I don't want to get further from that more immediate goal. If you see that being a challenge, perhaps we can talk about it tomorrow?

I'll for sure change these variable names and try to add some kind of documentation, somewhere. I suppose it's time we started having some.