Get rid of the class based task definition in favor of decorator-based
task declarations.
Doing so, we can get rid of core/tasks.py
Task names are explicitely set to keep compatibility with task
definitions in schedulers' database.
This also add debug statements at the beginning and end of each lister
task.
Streamline production of new listers by aggressively moving core
functionality into progressively inherited (A->B->C) base classes
with the transport layer abstracted.
This should make common individual forge listers straightforward to
produce with minimal customization. Github and Bitbucket listers
can be used as examples of the indexing type.
