Update slider and gallery to work with picture element.

I think this would be very reusable in areas outside the slider, and it definitely feels like it doesn't belong here. 

```
$img = $("<img src='http://images-resrc.staticlp.com/S=W1000M,H700M/http://media.lonelyplanet.com/assets/sloth.jpg' />);

resrcHelper.get($img); // http://images-resrc.staticlp.com/S=W1000M,H700M

resrcHelper.strip($img) // http://media.lonelyplanet.com/assets/sloth.jpg

$newImg = $("<img src="http://media.lonelyplanet.com/assets/a-better-sloth.jpg' />");
$newImg2 = $("<img src="http://media.lonelyplanet.com/assets/an-even-better-sloth.jpg' />");

resrcHelper.copy($img, [$newImg, $newImg2) // [http://images-resrc.staticlp.com/S=W1000M,H700M/http://media.lonelyplanet.com/assets/a-better-sloth.jpg, http://images-resrc.staticlp.com/S=W1000M,H700M/http://media.lonelyplanet.com/assets/an-even-better-sloth.jpg]

```