Refactor out SCXML-specific code

[jbeard4]

Generally speaking, SCION-CORE is meant to be a generic state machine interpreter. SCXML-specific properties should be passed in as arguments, either via opts, or embedded as a property on the SCXML model. This keeps the footprint small and the logic simpler.

I would suggest that the SCION compiler should embed the ioprocessors that it supports on the model. We should then move this code out of SCION-CORE and into the SCION scjson-to-module compiler: https://github.com/jbeard4/SCION-CORE/blob/master/lib/scion.js#L42-L46

Another example of this is _sessionId. Rather than generating it internally, we will write a library to do SCXML communication, and pass that in as an argument.