title: Hash param is missing in payU payment gateway android integration
body: <p>I need to integrate payU payment gateway in my android app. But when app is trying to to get hash key it gives me error saying that </p>
  
  <p>Hash param is missing </p>
  
  <p>In demo app there are two option to generate hash </p>
  
  <pre><code>if(null == salt) 
          generateHashFromServer(mPaymentParams);
      else 
          generateHashFromSDK(mPaymentParams, intent.getStringExtra(PayuConstants.SALT));
  </code></pre>
  
  <p>In demo app there are note saying that hash key generation should be done 
  on server side </p>
  
  <p>so I am passing salt as null</p>
  
  <p>but Now the question is Which server url I have to use to generate hash? 
  Demo app is using this url 
  <a href="https://payu.herokuapp.com/get_hash" rel="nofollow">https://payu.herokuapp.com/get_hash</a></p>