Expose serialization via JSON.

Expose serialization via JSON.

The last of Tom's work is now integrated with all URLs that can will
respond with JSON. Unfortuantely browsers (I'm looking at you Chrome)
will cache per URL rather than URL + Content-Type so if one requests
JSON for /repo/sha1/commit then navigates to said commit the browser
will served the cached JSON rather than HTML. As such JSON is also
exposed with the .json extension allowing JS to talk JSON without
messing up certain browsers' (i.e Chrome's) caching.