Audio playing, content logging

Putting this up for some preliminary review and inspection.

Not complete yet, but when complete:

Fixes #2387, #2396 takes a stab at #2394, #2395, and provides some required backend for #2386.

Summary of changes:
* Add audio-js.
* Create view and template for audio playing.
* Add appropriate code to sidebar navigation to instantiate audio view.
* Creates generic content and contentlog Backbone models.
* Creates generic contentlog Django model.
* Creates tastypie endpoints for content and contentlogs.
* Creates api_view mirroring video and exercise for contentlogs (should clean these all up to use tastypie).
* Creates methods for querying data and stamping content availability on disk, copying current video practices.

@jamalex @aronasorman comments on approach, and suggestions for streamlining and cleanup all appreciated!
http://jsperf.com/underscore-extend-vs-jquery-extend says _.extend is faster presumably because (as noted by http://techiella.wordpress.com/2013/04/02/underscore-js-extend-vs-jquery-extend/) _.extend only does a shallow copy of nested objects.