Two new commands: dump and restore

Hi,

I've implemented two new commands: `postgresql:dump` and `postgresql:restore`, which dumps/restore databases to stdout/from stdin or can be redirected to use files. The command `postgresql:restore` could also be used as a shortcut to a `psql` shell logged with the app's database credentials. Both commands depends on `pg_dump` and `psql` utilities, found in the package `postgresql-client`, and can be executed either locally or via SSH.

The columns shown on help were also aligned to use the same spacing of the other entries in `dokku help` itself.

Regards,
Tiago.