return the socket on execute

```docker_exec -it``` is not supported by docker-py. Currently, docker-py only supports reading of the response of executing a command from Stdout/Stderr. 

However, one cannot attach to the Stdin. 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.

Notes: The fake_api.post_fake_execute_start has an object called response. The third value in that tuple breaks docker's defined multiplexing format. The given amount of data  is 13 bytes, but it is set to 12 bytes. This PR also includes a fix for that.