Support JSON-based API for accessing contribution counts

[Aaron1011]

It would be nice if thanks supported retrieving the raw data through a JSON endpoint (or any other kind of structured format). This would eliminate the need to parse the rendered HTML in order to access the contribution data programmatically.

[steveklabnik]

This makes sense. I'd like to hold off for now though; the main crate is undergoing a lot of change, or at least, will be.

http://jsonapi.org/ is what I'd plan to expose.