* Allow users to update their Account in the API

It would be nice for API clients to be able to allow users to update
their accounts without having to wrap Mastodon in a web view. This patch
adds an API endpoint to let users submit a PATCH for their account.

Signed-off-by: David Celis <me@davidcel.is>

* Add /api/v1/accounts/update_credentials to the API docs

Signed-off-by: David Celis <me@davidcel.is>

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.