Add Rehearsals' API controller.

Includes scaffolded methods, plus the beginnings of a test suite for the
controller.
First test: return 201 Created.

Return the appropriate 201 Created HTTP response if everything goes ok.
Second test: return the new rehearsal's location.

Set the Location header in the response to the URI of the newly-created
rehearsal.
Indicate a bad request if the request body is null

Return a 400 Bad Request HTTP status code if the supplied request body is
null: we can't do anything with a null body.