Create Makefile

Add a Makefile to clean up installation procedure, making it possible to package plank-themer!
Create plank-themer.sh

Add a script to install the dockitem without using any root permissions.

The script can be run from the command line or from the "Plank-themer" menu item.
Merge Makefile and plank-themer.sh

There should be a way to do this on github's web interface.