Clone specific repositories as well

This will add a functionality to clone specific repositories
from ownCloud organization.

It also generates an error if the core environment is not set up.

To use: `python3 -m ocdev setup <repositoryname>`

The conditions for base and core are added because otherwise
the setup command could be used only once
Do you mean this? 

https://github.com/owncloud/ocdev/pull/1#discussion-diff-17161765

As you suggested, we do not need to do the same check again.