Feature/username exists check

* Added usernameExists method
* Added constraint to username	
* Added constraint to username	
* Added username check to register	
* Added username check to setUsername	
* Added usernameExists API method
server_error or database_error? 
https://github.com/Islive-io/fortress/pull/39/files#diff-27f41a975a1c4677af1ef5b7797e699eR16