Make start/end date fields store datetime objects internally

Bug fix:

* When setting start / due dates, the LMS would expect the `start` and `due` fields to contain `datetime` objects instead of strings.  This would cause a 500 error when accessing courseware for `OpenAssessment` problems that had start or due dates set.

* I have added a `Date` field to XBlock in this PR: https://github.com/edx/XBlock/pull/193
This will need to be merged, and we'll need to update the version of XBlock used by edx-platform.

* `OpenAssessmentBlock` now uses the `Date` field instead of `String`.  I've verified that this fixes the 500 error.

I really should have caught this when testing the original deadlines PR :(

@stephensanchez