Factor exceptions into exceptions.rb
Raise Celluloid::TimeoutError on mailbox timeout

Previously mailboxes would simply return nil upon timeout. This made it
difficult to disambiguate when timeouts occurred versus other events.

This changes mailboxes to raise a TimeoutError when timeouts occur.
Merge pull request #368 from celluloid/mailbox-timeout-exceptions

Raise Celluloid::TimeoutError on mailbox timeout