Add attach api call for containers

I have add the attach api call for containers : http://docs.docker.io/en/latest/api/docker_remote_api_v1.8/#attach-to-a-container

By default it streams everything (in default docker api it streams nothing) because i think when using this function user wants to have a stream of the output of container.

This function needs a callback for interacting with container output (explained in the docblock).