allow automatic collecting & grading of homework at the due date & time

[DrXyzzy]

This might be two options for a homework assignment:

1. Collect from students when due.
2. If 1) is enabled and assignment uses nbgrader, auto grade after collecting

Questions:

• what to do if work was already collected / graded by instructor before due date
• what to do if instructor wants to manually collect again after auto collection
• what to do if instructor wants to manually augment or override auto grading

Requested by Alan Levy and Siqi Jiao.

[williamstein]

dup of #109