code: public static Observable<BackendUser> signUpInBackground(String username, String email, String password){
          return getAM().signUpASync(new SignUpCredentials(username, email, password));
      }
docs: Perform asyncronously sign up attempt.
  @param username user name user will be identified by.
  @param email user email address
  @param password user password
  @return login results as observable.