Implement attach_websocket() for attaching with WebSockets instead of HTTP streaming
Move creation of websocket connection to its own method, so subclasses can override
Support https:// URLs (replacing with wss://) when attaching websocket
Use websockets by passing 'ws' param to attach_socket
Merge branch 'attach-websocket' of github.com:aanand/docker-py into aanand-attach-websocket

Conflicts:
	docker/client.py
Don't import websocket-client if environment is python 3 (not supported)