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.
We have Instrument and Facility models now.
Please use them instead, so we are not duplicating functionality in parallel ways.