Redesigned Comments API

Accessing comments is now done as a subset of a spark or idea, to list all the comments of a specific spark or idea, or to create a comment which automatically associates with a given spark or idea.

Whereas before, one would create a new comment by sending a request to `/api/v1/comments.json`, with a given spark or idea as an attribute passed in, instead a request must be made to `/api/v1/sparks/1/comments.json`, which will add a comment to the Spark with an id of 1. The same goes for ideas.

Sam, I hope this doesn't mean you have to change too much code on the android side, but I think this system makes for a much cleaner API.