FAKE should select the correct msbuild, based on VisualStudioVersion parameter

[hmemcpy]

When building Visual Studio extensions, msbuild version has an important significance. Each VS version has its own msbuild, and FAKE should invoke the correct one.

Right now, there's a very simple lookup for an installed MSBuild version. While this may work well for most C#/F# projects, it doesn't work with version-specific projects, such as Visual Studio extensions.

VS-specific FAKE scripts pass in the VisualStudioVersion parameter (either taken from the environment variables of the Developer Command Shell, or a default value) - see https://github.com/fsprojects/Paket.VisualStudio/blob/master/build.fsx#L105

FAKE should consider VisualStudioVersion parameter, if exists, when looking for the location of msbuild.exe