Display DHT statistics to the HTTP user.

Display DHT statistics to the HTTP user.

Adds 2 new interfaces to the DHT implementation:
 * DHTStats: DHT that supports gathering statistics
 * DHTStatsFactory: DHT that supports creating it's own HTTP server
apt_dht_Khashmir supports IDHTStats and formats stats for HTML.
apt_dht_Khashmir also supports IDHTStatsFactory if twisted.web2 is found.
Main script passes the DHT class to the main program.
Main script (DHT only) starts the stats factory if the DHT supports it.
Main program retrieves statistics from the DHT when asked by the HTTPServer.