Mailbox configuration for routes under BalancingPool - Issue 3964 fix

Fix for bug described here - https://www.assembla.com/spaces/akka/tickets/3964.
  * Ability to specify mailbox for routees under balancing pool in deployment configuration has been added
  * Correspondent tests added too
  * BalancingSpec fixed in section "deliver messages in a balancing fashion when defined in config"