Changes to TastyPie API required for MyData

No new API requirements have emerged from MyData for a while, so I think it's about time I submitted a pull request.  Carlo has made similar changes to the API for ObjectACLs - I don't think he's submitted a pull request yet.  This pull request supersedes the old one which added functionality to the API for creating user accounts.  That functionality is no longer required - users are being told that usernames must exist in MyTardis already, otherwise their data only appear in the default facility account associated with the MyData instance.