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!
Some additional top-level fields that come to mind as being useful: `content_type` (i.e. video vs audio vs pdf, etc), `progress` (integer? e.g., to store a percent?), `time_spent` (as a float, in seconds?), and perhaps something to do with `source` (e.g. WCH app vs someone else's, so we can run stats on that more easily).