New Tutorial style

[Jonathon Jongsma <jonathon jongsma gmail com>]

It looks like Murray has uploaded the new version of the gtkmm
tutorial to the website
(http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/index.html). 
It's mainly just an update of the style of the tutorial.  My goal was
to make it more attractive and at the same time more readable than the
current one.  I'd be interested in hearing any comments / suggestions
/ criticisms about it.

Thanks,
Jonner