Create terms API (CANVAS-187)

Adds a terms endpoint to the SFU v1 API:

/api/v1/terms - retrieves all terms
/api/v1/terms/nnnn - retrieves a specific term, where nnnn = a PeopleSoft term code (e.g. 1137)
/api/v1/terms/current - retrieves the current term
/api/v1/terms/next[/n] - retrieves the next term(s) -- /n is an optional limit (defaults to 1)
/api/v1/terms/previous[/n] - retrieves the previous term(s) -- /n is an optional limit (defaults to 1)