Folders

To be merged **after** Dan-Piker/Plankton#5 has been merged. If you're happy that is!

The only commit that is important to this PR is 0b49a35. (The rest will disappear when mesh-ops gets merged).

The addition of the `lib/` folder makes cloning and building this project *much* easier:

1. Clone.
2. Copy `Rhinocommon.dll`, `Grasshopper.dll` and `GH_IO.dll` to the `lib/` folder (and their XML documentation too if you want intellisense).
3. Build.

Previously I would have to edit `PlanktonGh.csproj` to add the paths to assemblies as they are on my machine and then try to remember not to commit it along with any other changes!