edx/edx-tim — pull request #58 by wedaly (2014-02-28 00:40:32+00:00)
Title: Will/fix preview errors

Fixes for two errors that occur when viewing the XBlock in Studio preview:

1) `DoesNotExist` error caused by looking up `StudentItem` with `student_id` set to `None`.
2) `ValueError` caused by trying to parse timestamp with milliseconds (which is the default value set by `isoformat`)

@ormsbee