Allow users to stop event propagation for nested DOM elements

Hey Brian, 

In [#65](https://github.com/briangonzalez/jquery.pep.js/issues/65), we talked about how to stop event propagation, because `ev.stopPropagation()` in the drag callback doesn't work. 

I've been thinking about the best way to do it, and even though stopping propagation by default is likely the behaviour that you'd want in most situations, it's the opposite of normal event behaviour — so it seems like it would be best to have propagation on by default and then provide a way to turn it off.

This seemed like the simplest and most transparent solution to me. Let me know what you think :)

By the way, I ran the grunt task and it passed both JSLint and the test suite.