Add backup task to :up:db

[EvanK]

Currently, from what I can tell, running genesis:up (or genesis:up:db) overrides the staging/prod db without generating a backup: https://github.com/genesis/wordpress/blob/master/deployment/lib/sync.rb#L123-L138

Right now, we've got a "missing posts" issue with nursing.org staging. I haven't been able to to pin it down to any :up commands being run, but it does seem like a useful thing to create a backup of the staging db before overriding it with local. Two ways we could do that:

1. Create it on the server in /#{deploy-branch}/backups/ (something already created a few .sql files there for nursing, but I'm not sure what) - we'll want to figure out how to keep just a few to prevent server bloat with sql dumps.
2. Create it locally on the dev's machine (using the existing :backup task w/no modification)

Thoughts?

[EvanK]

This should generate a locally stored backup of the remote server's database before syncing the local db up to the remote. I've tested this on a genesis site, but I'd welcome anyone else to doublecheck :D