HttpUtility.UrlEncode => Uri.EscapeUriString

* HttpUtility not included in PCL
Replace Header.Add with indexer access

For some reason, PCL only supports indexer access and
has no .Add () method
Remove crypto classes for OAuth

* Use PLAINTEXT OAuth only since PCL has limited crypto
* This is non fatal, as we (a) rely on SSL and (b) Rainy doesn't
  check for signatures and (c) signatures don't add security if
  the ConsumerKey is publicly known as in our case ("anyone")
Replace HttpWebRequest with HttpClient

* PCL has no System.Web