Removed portable tests proj in favor of existing

+ Removed Nutritionix.Portable.Tests from solution and moved tests
    to existing project.
+ Modified test project reference to use Nutritionix.Portable,
    including making internal classes visible from the latter.
Updated JSON.NET reference

To match the version being used in the new portable assembly.
Fixed null reference in UrlEncode

HttpUtility class I added was not correctly handling a null/empty
string value being passed to UrlEncode.
Fixed exception test failing

Unit test for HttpException was still referencing System.Web. Updated
using to Portable.System.Web.