Added '_exited' property to be used for providing better debugging for machine implementors (i.e. 'noticing' if exits are called twice or more than once exit is triggered, etc).  Also added some comments and comment TODOs to the constructor explaining all of the private state variables.
Fix npm test script
Make required inputs distinguishable in console output
Fix logic error w/ when _exited was getting set.
Added note about _exited not necessarily referring to the exit handler callback that was triggered; rather it is the name of the exit that the machine's 'fn' called internally.