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).
@Antaris @coxp: Why is this test here? When someone sets "TemplateBase" as the template base class and then sets a model with `@model` it should throw a exception in my opinion. In the previous code (before this patch) when someone sets a non generic base type "MyBaseTemplate" and a `@model` directive we would generate "MyBaseTemplate<Model>" even when this is invalid. So the reason it works with "TemplateBase" is actually a bug?