Add lookup support to actors (through the agent method select)

API changes in the agent:
1) Add select
2) rename stop_actor_by_id to kill

Agent tests are fixed acwqcordingly
Fig get_deafult_scatter_limits big

Both actors and agents classes had the same method name get_defualt_scatter_limit,
but with differenct arguments.
Add lookup support for actors
Add warning when blocking call is invoked and greenelets are disabled
Fix typo in documentation, add explanation for new features (lookup, kill)
Fix tests for actor stop