fixed migration issue

added a default to fix the lms_useritem.created_at field

the symptom was:

    Running migrations for lms:
     - Migrating forwards to 0002_auto__add_field_itemcomment_content_html__del_field_trail_created_by__.
     > lms:0002_auto__add_field_itemcomment_content_html__del_field_trail_created_by__
    FailedDryRun:  ! Error found during dry run of '0002_auto__add_field_itemcomment_content_html__del_field_trail_created_by__'! Aborting.
    Traceback (most recent call last):
      File "/home/yoni/Personal_projects/hack_for_israel/local/lib/python2.7/site-packages/south/migration/migrators.py", line 170, in _run_migration
        migration_function()
      File "/home/yoni/Personal_projects/hack_for_israel/local/lib/python2.7/site-packages/south/migration/migrators.py", line 59, in <lambda>
        return (lambda: direction(orm))
      File "/home/yoni/Personal_projects/hack_for_israel/hackforisrael/src/lms/migrations/0002_auto__add_field_itemcomment_content_html__del_field_trail_created_by__.py", line 36, in forwards
        keep_default=False)
      File "/home/yoni/Personal_projects/hack_for_israel/local/lib/python2.7/site-packages/south/db/sqlite3.py", line 25, in add_column
        raise ValueError("You cannot add a null=False column without a default value.")
    ValueError: You cannot add a null=False column without a default value.