model-level implementation of the task progress feature
implemented AJAXy updating of document done-ness in task view with rudimentary visual feedback
First stab at showing progress in task list
handling nil values in done-ness and avoiding division by zero
Progress column now sortable in both directions for editors
NOTE: the whole sorting behavior is (and has always been) a mess.  Sort by state or document count doesn't work at all in the editing tasks view.
Merge branch 'task_progress'

Conflicts:
	db/schema.rb