* lib/rupnp/ssdp/search_responder.rb: Create a module to be included
  for MSearchResponder and future USearchResponder.
* lib/rupnp/ssdp/msearch_responder.rb: include SSDP::SearchResponder.
  No more defining #receive_data and #send_response.

* lib/rupnp/ssdp/usearch_responder.rb: new class to handle unicast
  M-SEARCH requests.

* lib/rupnp/device.rb: #start_ssdp_server and #stop_ssdp_server
  written.