doc: Added proper PEP-287 docstrings for lumberjack.lumberjack

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
doc: Added PEP-287 docstrings for lumberjack.*.

* Also got rid of the useless Filter class.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>