Add the Qpid::Proton::Messenger.on_receive event hook.

This method is invoked when a message is received on the Messenger.

The fire_on_receive method is invoked by the event driver when a message
is received.
Added method stubs for starting/stopping the event driver.

The stubs simply raise an exception when called. They will be overridden
by drivers to perform actual useful functions.
Added Event Machine as an event driver.
Updated the README.md file with a little more detail.