Added an image export plugin, with a gallery demo and a basic test.

The auto_test is currently failing, as I'm unsure of how to make the plugin from extras available to the test.  Let me know what the best way to fix that would be, plus any other suggestions...

The code for the plugin itself is taken from http://cavorite.com/labs/js/dygraphs-export/ Happy to fix that however you see fit if necessary, I understand the code is a bit dated.