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.
I'm happy to use the new instrument and facility models, i.e. add API access for them and modify MyData to make use of them.  But they will only provide part of the functionality MyData needs here (according to our clients' preference for defining a default experiment).  We could replace the data owner and date collected parameters with new fields in the Experiment model, but we'd have to be careful to avoid confusion between the initial ObjectACL owner (which could initially be the "mmi" faciliy role account) and the data owner (the researcher who will be granted ownership to the experiment after it is created by the facility role account).  And similarly, we would have to avoid confusion between the time at which the experiment record is created in MyTardis ("created_time"), and the date on which the data was collected on the instrument.

In this case, I would prefer to have the instrument model available as a field of each experiment record, instead of each dataset record.  Maybe we could have both, but allow the instrument field in the experiment model to be NULL (or some "special" value) for experiments which span multiple instruments?