Make DateTime field more permissive

Further testing has revealed that Studio does directly manipulate the `start` and `due` field JSON representations.  This causes the edx-tim XBlock to raise a `ValueError` when it inherits these dates from a parent XBlock.

Given the tradeoffs, and the tight timeline for getting edx-tim released, I think using `dateutil.parser` is a good compromise.

@nedbat @ormsbee