Initial Razor 4 support.

- Razor 4 nuget package is not signed so we also do not sign our assembly in this case
  -> Isolation is not working
- No VB support in Razor4 (removed when compiling with RAZOR4)
- Razor4 is no longer using CodeDom, so ICodeInspector is no longer available (with RAZOR4). To provide something similar we need Roslyn.
- Custom constructors are also not working anymore (we need roslyn here as well).
- Solution now defaults to Razor4 (when opened in Visual Studio).
- We now build 2 nuget packages: 3.5.0-beta1 (razor2/3) and 4.0.0-beta1 (razor4).