behaviour with Turbolinks enabled, optimizing Rabl for production

and atom feed generation to some extent, as well as the way mentions are
stored

relations on Status#mentions

simplify FanOutOnWriteService, add /api/accounts/lookup method

POST /api/statuses                  Params: status (text contents), in_reply_to_id (optional)
GET  /api/statuses/:id
POST /api/statuses/:id/reblog

GET  /api/accounts/:id
GET  /api/accounts/:id/following
GET  /api/accounts/:id/followers
POST /api/accounts/:id/follow
POST /api/accounts/:id/unfollow

POST /api/follows                  Params: uri (e.g. user@domain)

OAuth authentication is currently disabled, but the API can be used with HTTP Auth.