Add docstring for BaseImporter
Add docstring for BaseImporter

It is getting difficult to understand how importer work, to fix that, added
docstring explaining whole importer workflow.

python-dirbtuves/akl.lt,python-dirbtuves/akl.lt,python-dirbtuves/akl.lt,python-dirbtuves/akl.lt,python-dirbtuves/akl.lt