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!
Sounds good! And we can continue adapting these up through the release as needed, so this is great for now.