Process-to-WAMP example

I've been asked whether we could add an example how to communicate with a forked process via WAMP - eg expose the functionality of an executable that is then started dynamically.

There are 2 scenarios here:

1. the executable should be started, left running and communicated with over pipes. A WAMP session then exposes eg a procedure, and when that is called, some communication over (the existing) pipes happens
2. the executable should be started "one shot", that is, upon an incoming call to a WAMP registered procedure, the executable should be forked (presumably with command line args and/or stdin derived from the WAMP call parameters) and waited for exit.

This could reside as a new example here: https://github.com/crossbario/autobahn-python/blob/master/examples/twisted/wamp/app/process2wamp

Start by reviewing the existing examples/twisted/wamp/app/process2wamp location and the surrounding Twisted WAMP examples. Define how the example should demonstrate both a persistent fork communicating over pipes and a one-shot executable invoked from a registered procedure; done means the example clearly covers the requested process-to-WAMP interaction.