add smart_render() to output for production/debug
fix subscr_failed IPN events not being accepted

Indicate the datetime format of all DateTimeFields so they can be
correctly parsed. This change is made in particular for retry_at. When
users create a subscription which later fails an instalment, paypal
issues an IPN event with txn_type='subscr_failed' which indicates the
date of the next attempt. This event failed parsing by django-paypal,
and gets stored into the database with raw content and a flag. This
fixes the issue.