Let the send_reminder of the fedocal_cron return the messages that were sent

This is useful especially for test and debug purposes
Allow the cron job to send the reminder to multiple addresses in one go

Relates to https://fedorahosted.org/fedocal/ticket/134
Add a dedicated unit-test file to test the cron reminder
Add mock to the list of dependencies as it is used now in the tests
Small cleaning
Do not store spaces in the email address specified for the reminders

Fixes https://fedorahosted.org/fedocal/ticket/134
Cron tests require a real sqlite db, but at the end go back to the in-memory one