Support requests >=2.1.0

Starting with requests 2.1.0 non-HTTP scheme URLs are not parsed anymore.
The net effect of this is that when using the unix socket in docker-py no
query string params are passed in the URL.  This change makes docker-py
internally use the scheme http+unix to indicate to requests that this is
still a HTTP URL and should be parsed.

Users of docker-py can still specify 'unix:' as the base_url.  The
following forms of base_url are accepted.

  http+unix://var/run/docker.sock
  unix://var/run/docker.sock
  unix:///var/run/docker.sock
  http://hostname
  tcp://hostname