Create UserNameConflictException Exception.

ACE-77
Define UserService interface and implement concrete MockUserService.

ACE-77
Moved UserNameConflictException to setUserName

ACE-77
Added DEFAULT_DOMAIN field which is appended to form a FQDN on login
setUserName now throws an exception if the user name is unavailable
Merge pull request #37 from ualerts-org/defineUserServiceInterface_ACE-77

Define user service interface ace 77
Merge remote-tracking branch 'origin/master' into serverBranch_ACE-76