Add New Relic extension and expand/refactor extension support.

Configurations can now include named event triggers that can be called in other parts of the code.
This is used here to allow the New Relic agent to be explicitly started and stopped (or else it
doesn't always report all its data.)
Provide an :amqp dispatcher type which automatically chooses hot_bunnies/bunny based on the Ruby engine.
Lifecycle improvements for actors and subscriptions.

hot_bunnies connections are now properly shut down, although the threads still seem to linger.
Celluloid actors are explicitly terminated when they're finished.
Rails generator to create script/woodhouse and config/initializers/woodhouse.rb.
Update and substantially expand README.
Remove redundant script/woodhouse-server.