Allow Roslyn to be compiled for MSBuild 4.0 so that it can be used in VS2012

The InvalidCastException happens because of a signature change in MSBuild, but other than that, the entire codebase seems to be compatible.

Provided that the solution is built with the following parameters, a VS2012 compatible build can be achieved:

msbuild Roslyn.sln /p:DefineConstants=MSBUILD12 /p:VisualStudioReferenceAssemblyVersion=4.0.0.0 /p:MSBuildAssemblyNameFragment=v4.0