Add HTTP Basic Authentication support for portable libraries

Issues:
- Portable libraries do not support CredentialCache and thus multiple credentials, which may be  
      necessary in case of proxies and credentials.

- Portable libraries do not support PreAuthenticate.

- Portable libraries do not support redirect follow-ups. Redirection has been defined for full profile, and 
      it will affect all and not just authentication scenarios.

- It's unclear if authentication function properly in Silverlight using the browser stack or should  
      "WebRequest.RegisterPrefix("http://", WebRequestCreator.ClientHttp)   
      WebRequest.RegisterPrefix("https://", WebRequestCreator.ClientHttp)" added.
      
       In that case also namespaces ystem.Windows.Browser and System.Net.Browser should be 
       opened and appropriate libraries added.

Some of these issues are concerned if one wants to handle authentication
in presence of redirects and how to build the case of OAuth.

Pointers:
- An F# OAuth2 library using authentication modules:
https://github.com/versionone/OAuth2Client.Net/blob/master/OAuth2Client/AuthModule.fs.

- An F# OAuth libary: https://github.com/Gab-km/OAuth-in-FSharp/tree/master/OAuth.

- An OAuth2 in C# https://bitbucket.org/takenet/takeio.client/src/55c15cbd40a2/CSharp/TakeIo.OAuth/OAuth2/.

- Google .NET OAuth2 API: https://developers.google.com/api-client-library/dotnet/guide/aaa_oauth.