Create metatag that will appear in Google+ descriptions

[GoogleCodeExporter]

At the moment when you share a slide-deck on Google+, the description takes 
whatever it can find first.

This defaults to 'testing' if you've no notes on your first slide, or to the 
actual presenter notes.

Would be great to have a meta tag of some sort to define what goes in the 
description, or even pull in by default something cool like a photo.

Original issue reported on code.google.com by mkear...@google.com on 18 Mar 2013 at 6:07