Add support for Django Signals

[Archmonger]

Discussed in #578

^{Originally posted by ZuluPro August 14, 2016}
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 lock/flush/etc before a backup or restore.