Fixes to project files to allow building with VS2012+VS2013 Build Tools (and Mono)

This is partial fix for #10. It allows the projects to build on Windows when VS2012 is installed and the VS2013 build tools are also installed; it may also allow the projects to compile on Mono (if they didn't already).

I've also modified the project file for ``FsLexYacc.Runtime`` so it targets the .NET 4.0 client profile instead of the full profile.

A full fix for #10 will require some tweaks to the FAKE build script, since that's what generates the .nuspec files for the packages (and ultimately creates the .nupkg archives). I'm not really familiar with FAKE, so I can't implement that piece of the fix.