Terminate transport child on terminate

[rockneurotiko]

This PR solves #39

On :normal stop, the child processes are not terminated. Let's trap the Socket process exit and manually stop the transport child to avoid dangling processes.

[fhunleth]

@rockneurotiko Justin has been away, so I figured that I'd led you know his plans before you put too much more time into phoenix_client. phoenix_client was originally written to support Nerves use cases, but it hasn't been a project that anyone on the Nerves core team has had any time or interest maintaining. I suspect the project will eventually be archived or maybe just live on with all issues/PRs ignored. We're looking at replacing all Nerves uses with https://github.com/NFIBrokerage/slipstream which is both actively maintained and fixes issues.

I saw that you've sent multiple PRs. If you have any interest maintaining the project, please let Justin know. Once he gets back, that might be interesting to explore.