pwclient: Support proxy configuration

pwclient: Support proxy configuration

The environment variables http_proxy and https_proxy can be used
to configure the HTTP transport.

The TCP connection is made with the proxy host, whereas the original host
is maintained in the HTTP POST URI via "handler" in "send_request".

The send_request() method of xmlrpclib has a different signature
and behaviour in Python 2 and 3.

Fixes #47

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>