* Redoing AddUser/AddUserRequestHandler with tests
Added first failing test
* Made test Test_UserDoesntExist pass
* Added failing test Test_UserAlreadyExists
* AddUser/AddUserRequestHandler now passes all tests
* Renamed User.Name -> User.UserName
* Refactored test:
 - Moved AddUserRequestHandler into production assembly and namespace
 - Extracted reusable DatabaseFixtureBase