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
Update Makefile to use NuGet to fetch FsLexYacc
With this change, cygwin and mono users will be able to automatically download fslex and fsyacc, removing the dependency on the old PowerPack on all platforms
Merge branch 'VS2013' of git://github.com/ad-l/FStar into ad-l-VS2013
Fix build under mono + update INSTALL
Merge branch 'ad-l-VS2013'