Make the BaseActor 'begin'/'finished' messages DEBUG.

The actors are verbose-enough, we don't need the base actor flooding
the user with INFO messages as well.
Add 'beginning X actions' log statement to the BaseGroup actor.

Since we hid the BaseActor 'Beginning' log statement, we re-add it to
the Group actors -- because groups are often used as organizational
units, and therefore you'd want to see when each group is starting.
Merge pull request #96 from Nextdoor/quiet_down_base_actor

Make the BaseActor 'begin'/'finished' messages DEBUG.