* lib/rupnp/control_point.rb:
  Replace USN by USN in logs

* lib/rupnp/service.rb: remove debug print
* lib/rupnp/event_subscriber.rb: Add EventSubscriber class. Used
  subscribe to event.
* lib/rupnp/event_server.rb: add EventServer class to receive all
  event notifications for a control point. This class dispatches
  notifications to subscribers.
* lib/rupnp/event.rb: Add Event class for holding event
  notifications.
* lib/rupnp.rb: add require for new classes.

* lib/rupnp/constants.rb: add constants for event subscription.

* lib/rupnp/control_point.rb: add management for event server. One
  unique server for all events for a control point.
* lib/rupnp/tools.rb: #snake_case correction to not add a '_'
  between an underscore and a upcase letter.

* lib/rupnp/device.rb: add #control_point attr_reader to retrieve
  control_point which controls this device.

* lib/rupnp/service.rb: add #device attr_reader to retrieve device
  which have this service.
  Add event subscription.