Courses API

This is a subset of #6109. The goal here is to trim the code down to exactly what will be needed by Insights and get it merged soon. We will revisit the other APIs soon after so that the edx-solutions fork isn't too divergent.

@mulby @mattdrayer @cpennington @ormsbee @dsjen @smagoun @shnayder @mikigoyal 
There should be a better way to organize the `patterns`. I am bit wary of all the calls to `.format()`. What about something like this?
https://docs.djangoproject.com/en/1.5/topics/http/urls/#including-other-urlconfs
We can group all the endpoints that start with `COURSE_ID_PATTERN`.