Add F# support to the Roslyn services

[IgorFedchenko]

Coming from PR #72 , we have an issue that when loading .net solution with MSBuildWorkspace class (from Microsoft.CodeAnalysis.MSBuild namespace), it does not recognize .fsproj files as a part of the solution.

That is because _workspace.Services.IsSupported(LanguageNames.FSharp) equals to false - that is, none of the registered host services are assigned to F# language.

While we have working hack here, which registers .fsproj files as C# project files (which allows to track project dependencies and other stuff), we need to do that in a "right" way and have some registered service assigned to F#.