Added support to delay sending of messages

Motivation
========
In some scenarios could be needed to wait some time before to send some messages.

Solution
=======
At the moment of adding a message to the queue, now you can specify an optional value of `sending delay`. which will be considered when the messages are sent by the management command `send_mail`

Other changes
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* unversioned *.pyc, .idea/*, *egg-info/
* generated south migrations