Get build working on Mono

There were two issues that I could find that prevented this from building with Mono (on OSX):

- The fsproj files were setup to use a local `lib/FSharp` but this didn't seem to work (doing things like trying to run `fsc.exe` directly instead of with `mono.exe`). I Removed `lib/FSharp` and references in fsproj files.
- The `nuget.targets` file was out of date and caused problems on Mono. A later version would fix it but since nuget has switched to the new restore workflow it's not needed anymore. So I removed the old nuget restore bits from the fsproj files.

I've tested this on OSX and Windows (cmd line and VS), looks ok.
@dsyme Have you tried this with Xamarin Studio or `xbuild`? I continue having to specify the `FSharp.Core` version when building in Xamarin Studio because `xbuild` complains that `$(TargetFSharpCoreVersion)` is not a valid version string.

**NOTE** I'm referring to running Xamarin Studio on a Mac. @7sharp9, would you be able to confirm? If someone can help me find a work-around, that would be very helpful.