Read outgoing mail settings from environment

These settings will define the email subject prefix, source address
and destination addresses for any notifications coming from a Django
instance. These can also be used without Heroku.
Dropping Python 2.5 support

It is about time to stop supporting this. So far it has been passing
but with Django 1.5 it is failing.
Merge pull request #21 from ferrix/identity

Read outgoing mail settings from environment