Events framework spike

This is a spike of an events framework for webmachine that is heavily based on ActiveSupport::Notifications. I'm not asking to merge this. (yet) ;)

I just want to get a discussion started to see if we want to have this in core. Having something like this would enable us to do things like logging, tracing, exception notifications, instrumentation and much more without implementing a separate subsystem for each of those. And users can use this to instrument their applications too.

There is an example app in `examples/` to see it in action.

    $ ruby -Ilib examples/events-framework.rb
    $ curl -i localhost:8080

**Caveats**

* No tests
* No documentation
* Not thread-safe
* Probably buggy

Please let me know what you think. Thanks!