Added portable version of WebApi.Hal for use as a HAL client API.

* Had to modify the Link and ReflectionExtensions classes as they used APIs not available in PCLs.
* Had to exclude the XML and JSON media type formatters
* All tests still pass when targeting the portable version of the API.
Updated portable library to target PCL framework profile 143 (WinRT, .NET 4.0.3+, Silverlight 4+ and WP8)

Various other minor changes to get tests to pass