Merge pull request #17 from jamalex/backbone

Backbone all the things (in particular, exercises)
move video_dict creation outside of the for loop
move handlebars imports to distributed/base.html
use handlebars for the formatting of a single playlist in the homepage
use kwargs['pk'] instead of kwargs['id'] so tastypie can properly
generate a resource_uri for playlists
use non-minified backbone to make debugging easier
only show playlists that are assigned to a student
partially port homepage playlists views to use backbone, and only show
playlist table when logged in as user
Moved style block CSS from exercise.html into khan-lite.css
Moved some dummy JS definitions from exercise.html into exercises.js
Dynamically add exercise-inline-style <style> block to page.
Moved khan-exercise.js reference down to the other JS file imports.
Removed obsolate TODO block.
Replace urlBaseOverride with direct building of the khan-exercises URL.
Move statusModel load deferral into ExercisePracticeView.
Use a deferred for indicating khan-exercises loading status.
Basic unstyled version of playlist view page with exercises loading.
Add a (hopefully unique) random ID to the video.js video element.
Avoid initializing video player more than once, but only within instance.
Remove duplicated javascript imports.