code: public RequestToken signup(BaasHandler<BaasUser> handler) {
          return signupWithStrategy(RequestOptions.DEFAULT,DefaultSignupStrategy.INSTANCE, handler);
      }
docs: Asynchronously signups this user to baasbox
  using provided password and default {@link RequestOptions}
  
  @param handler an handler to be invoked when the request completes
  @return a {@link com.baasbox.android.RequestToken} to manage the asynchronous request