Rename fields in CourseTaskLog, including a task_key column for indexing.  Use 'rescore' instead of 'regrade'.  Clean up task submission.

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.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.

Switch timed_module to store location, and use to navigate from timer when timer displays on non-exam course pages.

* reorder index fields

* Update all uses.

Added stores_state attribute for xmoduledescriptors as a way to declare if the module stores state in the database.

Make location into a named tuple, and use it more as a first class entity, rather than URL for identifying content

Make user profiles work again after the switch to an XMLModuleStore. Staff user histograms are still broken

Begin using a Keystore for XML parsing. Still broken: sequence icons, custom tags, problems, video js