* lib/rupnp/build_url.rb
  Create a module with a method to construct URL from a base and
  a target.

* lib/rupnp/base.rb:
  Create a base class for Device and Service. Base includes
  BuildUrl module. It defines HTTP_COMMON_CONFIG previouly
  defined in Device. Device#get_description is now in Base.

* lib/rupnp/device.rb: #get_description has moved to base.rb

* lib/rupnp/service.rb:
  Inherits from Base. #fetch now use #get_description from Base.