Blank NuGet package for Farseer Physics portable plus fixes

Updated .GitIgnore
Added NuSpec for FarseerPortable nuGet package (still needs author/owner/description updating)
Re-Added NuGet packages.config to solution as it was missing (stops NuGet package restore working)

Just edit the NuSpec with a text editor or use the NuGet package explorer GUI (http://docs.nuget.org/docs/creating-packages/using-a-gui-to-build-packages)
And add your own detail information.  Dll already packages for PCL's