code: public static RequestToken fetch(String username, BaasHandler<BaasUser> handler) {
          BaasUser user = BaasUser.withUserName(username);
          return user.refresh(handler);
      }
docs: Asynchronously fetches an existing {@link com.baasbox.android.BaasUser} from the server
  given it's username.
  
  @param username a non empty username
  @param handler  an handler to be invoked when the request completes
  @return a {@link com.baasbox.android.RequestToken} to manage the request