Configurable mailboxes 404

PR for #404 
`WithUnboundedStash` and `WithBoundedStash` now inherits from `RequireMessageQueue`
Some initial work on creating configured mailboxes through the `Mailboxes` class.
e.g. Required message queue semantics can be resolved now.

This also makes the behavior of stashing work as intended, messages are prepended on the mailbox message queue.
Just throw an ConfigException as the do in Akka JVM?