Updates for recent Django changes

There are three main changes here:

1. Added Django 1.7 migrations in parallel to South migrations (requires South 1.0+ or Django 1.7+),
2. Added default values to BooleanFields in compliance with changes in Django 1.6,
3. Using dateutil to parse the timestamps returned from Postmark, as the format they they return the dates in appears to have changed and this should be a more flexible approach.