Bump API to 0.3.1; add "Roles" endpoint

Wasn't sure this warranted a full point release bump (to 0.4) since it doesn't change any of the endpoints that the front-end team is already using. Although I would like to propose the [204 status code](https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#2xx_Success) for `DELETE` requests in lieu of 202s.

The "Roles" endpoint doesn't specify any search capabilities at the moment. I just wanted to get enough in the spec to build the model and controller in Rails.

I apologize in advance for the HTML tables. The [original Markdown spec](http://daringfireball.net/projects/markdown/syntax) (which is what Apiary uses) doesn't specify a shorthand for tables like [GFM does](https://help.github.com/articles/github-flavored-markdown#tables).