Floor boards add-on: random thickness

Acting on a question from Ákos I added a random thickness option to the add-on.

The new version is of course available on GitHub. (this is a single python file: select File->Save As... from your browser and save it somewhere as planks.py. Then in Blender, select File->User preferences->Add-ons->install from file... (don't forget to remove an earlier version if you have one).

If you select a non-zero thinkness each plank gets a random extra thickness up to the chosen maximum.

The option is located together with the other randomization options (which can be applied in parallel.)