XTMF2 User Object minor suggestions

Currently User objects in XTMF2 only have the "Admin" property (boolean). For future sake, it might be better refactored into a User which contains a set of roles -- even if there is just the one: admin. This pattern also interfaces with the sister project XTMF2.Web .

The other minor comment would be to rename Admin to IsAdmin to reflect that it is a boolean property (minor).
