almost complete rewrite of http_server module.

* now support asynchronous use like the other server modules by forking into the background.
* instantiate httpd object
* call httpd.serve()
* do other stuff
* call httpd.wait() or
* call httpd.shutdown() to shutdown prematurely
updated __init__.py.__all__

* remove LimitedHTTPServer and LimitedHTTPRequestHandler
* add HTTPConnectbackServer
Added pydoc comments to http_server.py
added support for document root to http_server

* if optional docroot is passed to constructor it becomes the server's document root.
* if docroot is omitted, the the current working directory becomes the document root.