Miscellaneous improvements.

* Use System.Web.Script.Serialization.JavaScriptSerializer instead of
  NewtonSoft.Json (just to avoid external dependencies).
* Target the full 3.5 profile instead of the client profile, since
  JavaScriptSerializer requires it (the nuget package was targetting the
  full profile already anyway).
* Add DecodeToObject() method to deserialize the JSON payload.
* Throw custom exception on invalid signature instead of
  InvalidOperationException.
* Add doc comments to methods.
* Add installation / usage details to Readme.md.
* Bump version number to 1.2.