Use standard hallo.js library distribution (v1.0.4)

Wagtail seemed to be using a self-built version of hallo.js with a few modifications to standard hallo.js plugin namespaces which make it impossible to load those plugins without modifying the wagtail source or packaging and including the plugin oneself. That seems overly restrictive and redundant. It's also a good idea to document the version of hallo.js being used so that custom plugins can be tested against it.

Since there's no pre-build hallo.js source provided and the diff is sort of huge, it's hard to tell if there were any other modifications to the hallo.js source shipped in Wagtail. Hopefully someone from Torchbox knows if this change is a big deal or not. This standard dist from the current master of the hallo.js project seems to drop in and work fine, though.

It also occurs to me that the custom Wagtail plugins for hallo.js are only provided as compiled JS. Having the original coffeescript files would be nice to have for maintenance purposes and as an examples for extending the editor.