Update to latest remote API 1.6

@bfulton @nahiluhmot

We'd been slightly out of date on a few fronts so this just took care of that.  My VCR pull just got merged so this is just pending a Gem release.

This that've changed:
- Docker::Container#attach now returns both stdout and stderr strings.  Since Docker has started to mux these together, the result is a Array of Strings for each stdout and stderr
- Docker#authenticate! works and uses creds
- Docker::Event has been added for streaming events
- Docker::Image#push and Docker::Image.create both take credentials in the form of a JSON AuthConfig object (https://github.com/dotcloud/docker/blob/master/auth/auth.go#L28)
- Docker::Image.build and Docker::Image.build_from_dir take a hash of options to specify query parameters (for tagging and forcing nocache)