Allow RESTful API queries to check if user exists, using non-admin account.
API now responds to instrument/owner/date query required by MyTardis Desktop
App prototype.
Fix for bug where concatenating the path_parts list with a directory string
resulted in the directory string being converted into a list of characters,
e.g. ["s","u","b","d","i","r"]
always_return_data=True allows us to get the JSON back when a new experiment
or dataset is created, instead of just getting an HTTP 201 status code.
ObjectACLs can now be accessed via RESTful API