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

update readme

modifications and exceptions
Add type hinting
add activity app to base, closes #3
some tweaks
do not change dir, just clone into current dir
bump version, new tool url
Create AUTHORS.md
Merge pull request #1 from owncloud/clonespecific

Clone specific repositories as well