Add touch device support

This adds touch support, because the dragging and dropping was not working on mobile devices. Uses [jQuery UI Touch Punch](http://touchpunch.furf.com/) to make this work.

I've tested and confirmed it works in:
* Mobile Safari (iOS 8) on iPad with xblock in workbench
* Mobile Safari (iOS 8) on iPad with xblock in LMS
* Desktop Safari 7.1 with xblock in LMS
* Desktop Chrome in LMS and workbench

Note: in order to connect to the devstack from an iPad, I had to modify `lms/envs/devstack.py` on the computer and set `SITE_NAME = '192.168.1.xx:8000'` rather than `SITE_NAME = 'localhost:8000'` - otherwise the XBlock wasn't loading correctly.
@e-kolpakov +1 - that can be helpful to add to the newcomer documentation, feel free to send a PR there, they are always appreciated and useful : )