allow input elements within draggable element to work properly

I'm liking pep! .. but when we have input (or other interaction elements) inside the element we want to drag around, it seems we have problems with pep gobbling focus.

This patch prevents pep from obscuring the behaviour of input elements inside the draggable. demos/containing-input-elements.html shows it at work. The fix basically prevents propagation of the events that are of interest to pep if they first hit an input element. 

Couple of things to note about the pull request:
* I haven't compiled dist/ sorry [npm install is currently giving me indigestion, like with https://github.com/mishoo/UglifyJS2/issues/297]
* I'm new to the jquery.pep code, so I may not have picked the right spot in the code to make the intervention or done it in the prefered - appreciate your feedback