Adding the #pool method to ActorServer.

Adding the #pool method to ActorServer so the actor server can creates a
pool of actors.

-----------------
Example:

class ActorA < Actor
end

class ActorB < Actor
end

class ActorC < Actor
end

actor_server = ActorServer.new('localhost', 8888)
mailbox_a = actor_server.pool('foo', ActorA) # defaults to 1
mailbox_b = actor_server.pool('bar', ActorB, 1)
mailbox_c = actor_server.pool('baz', ActorC, 5)

actor_server.run!
----------------

Bonus: Fixing the actor server specs - using the method #run! (from
Runable) instead of #start method (which doesn't exist anymore).