Adding a Zepto compatible version of Backstretch.

I was using [Zepto](http://zeptojs.com) on my current project and was in need of Backstretch. Turns out it requires a bit of work to run with Zepto instead of jQuery, so I made a new version called `zepto.backstretch.js` which is fully compatible.
I've also updated the `README.md` file with a few instructions.
Here's a fully working example: http://codepen.io/rafaelrinaldi/full/gkqHa