Changes to allow rgsimulator to be extend via inheritance

[treacheroust]

See rgsimulatorExtendedExample.py for how to do this. This provides a way to add behavior to rgsimulator without mucking with the base rgsimulator code. For example, this can be used to add new key binding with custom behavior.

The main changes are:

1. move self.UI.run() out of the constructor, so the class can be extended before the UI is started.
2. the Simulator constructor is now passed the uiClass, so a custom UI can be instantiated, instead of explicitly instantiating the base SimulatorUI class.

Apologies if I've screwed something up, I'm relatively new to python and GitHub, and this is my first attempt at contributing to a project.