fix crash in peer eval xmodule

As documented in Jira LMS-806:

Steps to reproduce:
* as one user, for a peer evaluated problem, enter something that needs
  peer grading
* as another user, log in and fire up the peer grading panel. You should
  see something that needs attention, a yellow exclamation badge
* clicking on the "do grading" panel will fail with a 500, stacktrace
  attached.
* it's possible that the question where this is being done must first have
  professor calibration done in order for peers to be able to actually get
  work to grade.

Don Mitchell debugged this with me and saw some problems in the ways
that the peer_evaluation xmodule code was accessing dates. Instead of
using the normal lms accessors, by going straight into the _module_data
it would be bypassing some important code (caching?). Fixing this to be
more standard did the trick.
Merge pull request #547 from edx/sef/peer-eval-xmodule-date-error-master

fix crash in peer eval xmodule