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.
There are a few different ways in which MyData uses email addresses.  Here are some use cases:
1. A facility manager is using MyData on a microscope PC, and finds some failed uploads for one particular user ("jsmith"), so then they go to MyData's users view, to find out who "jsmith" is, and contact them by email.
2. A MyData user (who is not a facility manager) doesn't know if MyTardis will recognize their "jsmith" username (and whether it will be mapped to the correct MyTardis account), so they create a "jsmith" folder, and ask MyData to scan their data folders within that folder, then they look at MyData's Users View.  If MyTardis recognized the "jsmith" username, then MyData's Users View will populate fields other than Username, i.e. Name and Email.  Name could be ambiguous (e.g. "John Smith"), but email address shouldn't be.  If MyTardis didn't recognize the "jsmith" username, then they will see an error in MyData's Users View next to the "jsmith" username.  If we drop the Email column from MyData's Users' View, then it will only have two columns: username and full name (if found in MyTardis) or error message (if not found in MyTardis), which might look a bit empty.
3. MyData is deployed across multiple sites, some of which don't share the same LDAP system, and some of which don't support AAF.  MyData users want an easy way to label data as belonging to a particular user, so they put their data in a folder whose name matches their email address (yes, all OS's seem to support odd-looking email addresses for folder names).  The alternative is that users must have a MyTardis account created for them and know its username before they use MyData for the first time - a bigger hurdle in getting started.

Many of these use cases only require querying a specific username or email address and asking "Is there a valid MyTardis account for this username or email address?", rather than requesting a list of all other users' email address.  So we could implement some special authorization to prevent API users from downloading the entire list of users' email addresses.  MyData uses a facility role account to authenticate to MyTardis, so I would have no problem with assigning special permissions for that account to allow querying of email addresses, but not allow querying them for regular MyTardis accounts.  But we would need clear instructions on how to create this facility role account.

Hopefully this clarifies MyData's requirements a bit.