Update to be mono compatible.
Add the FSharp.Core portable files to the GAC during install

As a portable FSharp.Core has now been successfully compiling in Mono
for a while add the assembly to the GAC.  This is a precursor to adding
the final part in the F# addin for portable projects.
Merge pull request #271 from 7sharp9/master

Update Microsoft.Portable.FSharp.Targets to be mono compatible and add the portable Fsharp.Core to the GAC on install