Chunk sending

* We can limit the number of emails sended on every scheduler execution
* We can assign priority when sending emails

This way if we want to send 2000 mails we won't have problems with mail server. Also if an urgent mail must be send, with a higher priority we assure that it will be sent the next time scheduler is called.

We can pass the ```{'limit': X}``` inside context when calling scheduler for limiting to X the amount of sended emails in a row.