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.
Return a child with the given name for the given request. This is the
external interface used by the Resource publishing machinery. If
implementing IResource without subclassing Resource, it must be provided.
However, if subclassing Resource, getChild overridden instead.