Upload and download commands should be generic

As of today, only databases with SQLite files can have external metadata that is downloaded from the Idiap server and placed inside the package before getting it into PyPI.

This is method is also useful for packages which may include metadata that is too big to add into the git repository.

In this context, it would be useful to remove this (strange) restriction from this package and let packages decide what is metadata they want to have outside of their git repos. (Example: bob.db.cohface and bob.db.hci_tagging).