Make Mailbox#next_message private, closes #374

Calling #next_message without holding the mutex is unsafe. This method should not be part of Mailbox's public API.
Shut down mailbox after each example

This is needed for celluloid-zmq, since we want to be able to close the ZMQ context, which hangs if the Mailbox isn't shut down properly.
Prevent race condition in JRuby

JRuby is fast enough that sometimes the active thread manages to grab the same thread that the idle thread is using, which means the number of threads is not correct
We aren't using ThreadGroup anyway, are we?
Merge pull request #378 from jnicklas/jruby-stability

Jruby stability
Merge pull request #375 from jnicklas/private-next-message

Make Mailbox#next_message private, closes #374