HTTP tunneling proxy support

I have added support for HTTP proxies that tunnel HTTPS traffic after the
CONNECT command. Proxy configuration is taken from 'https_proxy'
environment variable.

It is easy to run duplicity behind the proxy:
$ export https_proxy=http://proxy_host:proxy_port/
$ duplicity --your_parameters cf+http://your_bucket

As support for tunneling is added in python 2.7 officially,
running python-cloudfiles in HTTPS mode with https_proxy set
triggers an exception.