Targeted .NET 3.5 for portability

Retargeting required small modifications of EVERYTHING.  Also had to
remove the nuget sources because xbuild doesn't play nice with the new
formats.

But now everything builds and runs with Mono!  To build, use xbuild
CNC_Machine.sln, and to run just use mono PathCAM.exe.  See the Examples
folder for a couple .dae files to load.