Nicer pyglet import error message if version not high enough

The version of pyglet that we need is not released yet. So one needs to
do "pip install http://pyglet.googlecode.com/archive/tip.zip".

Also added a note; I found that the pyglet process events function
does not really process paint events, but this might need further
investigation.
Forgot to set back the version number

I set it to 1.6 for testing the new behavior.
Merge pull request #173 from almarklein/improve-pyglet-version-message

Nicer pyglet import error message if version not high enough