add razor4 build (make it the solution default).
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.
- We now build 2 nuget packages: 3.5.0-beta1 (razor2/3) and 4.0.0-beta1 (razor4).
update to nunit 2.6.4.
test implicit implementation.
always build Release mode.
Merge pull request #204 from matthid/razor_4

Initial Razor 4 support.