return the socket on execute

```docker exec -it``` is not supported by docker-py. Currently, docker-py supports reading of the response on executing a command. 

However, one cannot attach to the Stdin and send data. This pull request is meant to address that.

The way I do it is by returning the raw socket to the caller of this method. The caller can then call read from Stdout/Stderr and write to Stdin using the socket.