Added support for postMessage's into any iframes for content that needs to do something when a slide is displayed and hidden.

If config.center is false, this change centers slides with the
'center' class.

Also add a description about the solarized theme.

use 'url()'-free path when specifying parallax image, refactor so that parallax is applied to background class, remove unused attributes #595

Fixed problem with parallax background  not working under FF