Add support for Django Signals

[ZuluPro]

Django Signals could be a good way to let developers interact with DBBackup's actions. I imagine the following:

• pre_dbbackup & post_dbbackup
• pre_dbrestore & post_dbrestore
• pre_mediabackup & post_mediabackup
• pre_mediarestore & post_mediarestore

Would help users make a lock or flush something before backups.

[Archmonger]

This feature has been added within #614