Render a request. This is called on the leaf resource for a request.
Render must return either a string, which will be sent to the browser as
the HTML for the request, or server.NOT_DONE_YET. If NOT_DONE_YET is
returned, at some point later (in a Deferred callback, usually) call
request.write("<html>") to write data to the request, and
request.finish() to send the data to the browser.