support for multiplexing log streaming in api v1.6

http://docs.docker.io/en/latest/api/docker_remote_api_v1.6/#attach-to-a-container

without this change, the AttachToContainer call will return random characters in the log stream. i am yet to figure out how to make the tests pass with this one:

```
--- FAIL: TestAttachToContainerLogs (0.00 seconds)
        container_test.go:516: Unrecognized input header
--- FAIL: TestAttachToContainer (0.00 seconds)
        container_test.go:568: Unrecognized input header
FAIL
exit status 1
FAIL    github.com/fsouza/go-dockerclient       0.026s
```