rm alias Mealy

because it is not much longer than MealyMachine
stop using OrderedDict

it only complicates things and surprises the user.
Ordering should be defined explicitly, using e.g. a list (possible future implementation).
rename variables
bugfix: use None as default arg value
add new arg masks to MooreMachine.add_outputs
rm NotImplementedError from MooreMachine

add bib references to Moore and Mealy machines.
extract _print_ports, _print_label from MealyMachine.__str__

so that they be used by MooreMachine as well
rewrite MooreMachine.__str__
add new function: moore2mealy