Update to Visual Studio 2013.

This commit changes several things:
 - Solution and project files are now compatible with VS2013
 - Upgrade FSharp.Core to version 4.3
 - Remove dependency on the fsyacc and fslex from the old PowerPack
 - Add dependency to the FsLexYacc NuGet package
 - Add auto-download of NuGet package
 - Add compile rules to build parse.fsy and lex.fsl automatically