Added a "database-export" method with a Database plugin class.

[r-martins]

With this plugin now its possible to dump a store database to a sql file using
wiz database-export
And it will ask (or get current store) host,user, db, etc and also ask if you want to compress the resulting file.

It needs to be added to autocomplete but I didn't find out how doing that.

[davidalger]

I just pulled this to test and found a few minor problems that really should be fixed before resolving the pull request:

1. When MySql encounters the error mysqldump: Got error: 2002: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2) when trying to connect it does not halt the process, it proceeds to asking me how to pack the resulting dump.
2. When answering Y to "Compact sql to .tar.gz file? (N):" the original dump should be removed after it has been compacted.

Can you make these changes? Assuming yes, I'll be happy to pull this in.