Allow editing of notes via the api

Adds the ability to edit notes using a POST to the existing GET endpoint for fetching an individual note.  The user editing the note must be the same one who created it.

Tested using curl:
curl -H "PRIVATE-TOKEN: <token>" -H "Content-Type: application/json" https://gitlab/api/v3/projects/51/merge_requests/11/notes/38 -d '{"body": ":thumbsup:"}'