Support for resource priority attribute postpone.

This commit adds support for the W3C's draft specification for the
postpone resource priority attribute. Any picture element with postpone
or postpone="true" will not be processed until it is visible on the
screen. Additionally, this commit eliminates the unnecessary loading of
IE8 backup images in browsers that support the full polyfill.
Add capability to watch picture elements.

The pictureTime polyfill can now watch the picture elements on the page
and run itself when it notices a change. If a picture is altered
programatically, then pictureTime will make sure it the images are still
correct.
Solve case where picture element has no sources.
Merge pull request #1 from natgeo/NGM-1001

Add capability to watch picture elements