Refactor Backbone Notifications

Models are extraneous for something as temporary as a notifaction -- this change
moves all the configuration into the views, and removes the models entirely.
Add underscore.string library

http://epeli.github.io/underscore.string/
Only show at most 300 chars in AJAX error notification
Removed unused test code
Merge pull request #199 from edx/db/notifications-refactor

Refactor Backbone notifications