Validate date/time settings when typed in directly.

The changeTime event isn't fired when the user types in the field, but
only when clicking on the time in the dropdown. I'd consider this a
timepicker bug, but for now we can just listen to the user typing in
the field and update the value (and thus validate) like we do with
other field types.
Merge pull request #403 from edx/peter-fogg/fix-stud-421

Validate date/time settings when typed in directly.