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.
`self.runtime.local_resource_url` appends `settings.SITE_NAME` from lms configuration to resource url. Devstack have `SITE_NAME` set to `localhost:8000`, which, obviously does not work when trying to access lms from mobile device. It shouldn't be a problem in production, since `SITE_NAME` should point to lms domain  name anyway, but it could make sense to either:
1. Use some other way of appending resources: namely, anyone that results in relative links for resources (I'm not yet aware of any, @antoviaque, @dragonfi, @aboudreault or @mtyaka to the rescue :))
2. Assemble some manual on making it available for external devices (in my case it was just setting `SITE_NAME` to IP of the computer running lms)