Issue74: support drag and drop of folder

- I implemented folder upload for my project and decided to contribute it back to dropzone.js
- It's tested on Chrome.
 - I believe Chrome is the only browser that supports folder drag & drop.
 - This feature was added to Chrome 21: http://updates.html5rocks.com/2012/07/Drag-and-drop-a-folder-onto-Chrome-now-available