Support for F# and VisualBasic project files

[mavnn]

This is a make or break issue for us, as we use a mixture of all three languages.

Having said that, I don't see any C# specific functionality in CsProjFile.cs so it shouldn't be a massive change. I'll try and have a look into what would be required.

[mavnn]

In fact, I'll need to do some testing but I believe it might be as simple as changing:

    public void ForProjects(Solution solution, Action<string> action)
    {
        var csProjSet = new FileSet { Include = "*.csproj" };
        var targetDir = Path.Combine(solution.Directory, solution.SourceFolder);

        _fileSystem.FindFiles(targetDir, csProjSet).Each(action);
    }

to:

    public void ForProjects(Solution solution, Action<string> action)
    {
        var csProjSet = new FileSet { Include = "*.csproj;*.vbproj;*.fsproj" };
        var targetDir = Path.Combine(solution.Directory, solution.SourceFolder);

        _fileSystem.FindFiles(targetDir, csProjSet).Each(action);
    }

if I'm understanding how FileSet works. We'd probably also want to rename the CsProj class at that point, of course!