'Configures the `AsyncHTTPClient` subclass to use.
``AsyncHTTPClient()`` actually creates an instance of a subclass.
This method may be called with either a class object or the
fully-qualified name of such a class (or ``None`` to use the default,
``SimpleAsyncHTTPClient``)
If additional keyword arguments are given, they will be passed
to the constructor of each subclass instance created.  The
keyword argument ``max_clients`` determines the maximum number
of simultaneous `~AsyncHTTPClient.fetch()` operations that can
execute in parallel on each `.IOLoop`.  Additional arguments
may be supported depending on the implementation class in use.
Example::
AsyncHTTPClient.configure("tornado.curl_httpclient.CurlAsyncHTTPClient")'

@classmethod
def configure(cls, impl, **kwargs):

    super(AsyncHTTPClient, cls).configure(impl, **kwargs)
