Added 'fast' option to WSGIRefServer to disable reverse DNS look-ups.

The default WSGIRefServer implementation performs a reverse DNS look-up for every log entry.  If rDNS is slow or down, this holds up every web serve - possibly as much as by the rDNS time-out of 5 to 10 seconds!

The following patch adds an option to easily disable rDNS look-ups and logging through the run() method, like so:

    app = bottle.default_app()
    bottle.run(host='0.0.0.0', port='80', app=app, fast=True)

This has proved very helpful for speeding up page serves for me.  I hope it proves helpful for others.