ActiveSupport::Notifications events framework spike

As promised in #78 this is an alternative events implementation spike backed by ActiveSupport::Notifications.

I put AS::Notifications behind an interface (Webmachine::Events) and made the backed configurable so AS::Notifications can be replaced with something else if needed.

The gemspec depends on active_support 3.2.x since 4.0 hasn't been released yet. The dependency should be switched to 4.0 as soon as Rails 4 is out because the 3.2 AS::Notifications isn't thread safe.

There is a new example (`examples/logging.rb`) that implements a simple logger based on the `wm.dispatch` event.

    $ ruby -Ilib examples/logging.rb
    $ curl -i localhost:8080

Please let me know what you think about this. Thanks!