Added support for Windows Phone 8

* Added solution and project files for Windows Phone 8.
* Added conditional exclude for setting AutomaticDecompression on the
HttpWebRequest since Windows Phone doesn't support this property.
* Added Windows Phone-specific async version of ForecastUIRequest.Get to
use WebClient.DownloadStringAsync (non-async not supported on WP8) and
JSON.NET for JSON deserialization since Windows Phone doesn't support
the JavaScriptSerializer.