Razor2

Implemented Razor v2 view engine. Closes #547

The implementation is compatible with existing Razor v1 engine. The project ServiceStack.Razor2 is a copy of the v1 code, modified to support Razor 2. Only a few files outside the project needed changes (specifically, Markdown engine evaluator assumed that namespace always equals to assembly name, which is not the case for the new razor2 assembly),  

New support for conditional attributes is lifted verbatim from NancyFx: attribution is in code comments, consistent with existing comments.

It removes the dependency on installed MVC projects as it uses `System.Web.Razor.Unofficial` nuget.

I've modified `RazorRockstars.Web` to use the V2 engine. Note that `Views/Shread/RazorPartial.cshtml` correctly resolves relative (`~/`) links without any extra magic.