Cherry pick change to url matching regular expression. Fixes LMS-37
Instrument sequentials with segment-io so we know when people switch tabs manually or using the arrows to navigate
Updated `diff-cover` to version 0.1.2, which fixes a bug
in which missing lines were over-reported
Incorporate Segment.io into Logger.log with whitelist
Add new Jasmine test to ensure Segment.io is called when appropriate
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
Define initial celery task on instructor dash.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.
Add task progress table to instructor dash.

Add call to MakoMiddleware() to initialize templates in celery worker server.

Pass task_ids by POST properties (as a list) to collect task progress status.
Pull task_queue.py methods out from tasks.py, to represent API calls
from client.  Tasks.py remains the task implementations running on the
celery worker.

In particular, move status message generation out of task thread to client side.
Refactor tracking and xqueue initialization to no longer require a request.  Get tracking logs to work.  Add transaction annotations.

Make sure to read only once from result objects when getting result status,
so that values are consistent.
Add additional background tasks: reset attempts, delete state.  Update
CourseTaskLog fully after task submission, so it works in Eager mode
(for testing).
Clean up task progress javascript.  Add before/after scores to tracking of regrading requests.
Add tests for regrading at capa_module level.
Add check for problems that (do not) support regrading.
Check descriptor to identify problems that don't support regrading.
Add factory for CourseTaskLog.

Add unit tests for regrading at courseware level (task and task_queue).
Remove delete button from instructor dash. (Also remove last-modified
column from task-in-progress table).
Add background task history to instructor dash (as table).
Add task-history-per-student button, and fix display of task messages.