Add launcher script

I tried to package this program in Ubuntu, and found that it has no
startup script. So I added a `colout` script that gets installed into
the scripts directory with the `setup.py`.
Call Python 3
Change License to GPL 3 only

It has been 3+ previously, but the remainder of the project is using 3
only.
Merge pull request #25 from martin-ueding/launcher

Add launcher script