Integrate AS::Notifications and use it to publish traces.

Adjust debugger example and add logging example.
Create a generic events interface backed by AS::Notifications.

Webmachine::Events implements the AS::Notifications API and uses
AS::Notifications as the default backend. The backend is exchangeable
with any class that implements the AS::Notifications API.
Switch backend from ActiveSupport::Notifications to AS::Notifications.
Merge branch 'master' into as-notifications

Conflicts:
	lib/webmachine/trace/resource_proxy.rb
Add specs for Webmachine::Events.
Documentation for Webmachine::Events.
Merge pull request #92 from bernd/as-notifications

ActiveSupport::Notifications events framework spike