Added Support For Allegrograph, Basic Auth, and Proxy

Added support for describing repository using content-type: "application/sparql-results+xml"

Added Basic Auth and the Ability to use a local proxy.

If there's anything I can do to improve the submission, please let me know.  I was unable to test the JSON code path for getting repository info since I did not have access to an rdf store that supports it.

Here's an example of how to use the code:

<code>
  # @example Connecting to a Sesame server using Basic Auth & local proxy
  #   url    = RDF::URI("http://host:port")
  #   server = RDF::Sesame::Server.new(url, {:user=> 'username', :pass => 'password', 
  #       :proxy_host => 'localhost', :proxy_port => 8888})
  #   repo = server.repositories['repositoryname']
</code>