wip: Allow listener to be subscribed to all instances of a class [#31]

* failing spec
wip: Allow listener to be subscribed to all instances of a class [#31]

* passing spec
* implementation
wip: Allow listener to be subscribed to all instances of a class [#31]

* remove "magic number"
* allow an array of classes to be passed to scope
wip: Allow listeners to be subscribed to all instances of a class and its subclasses [#31]

* class can be given as a class, symbol (camelcase) or string, or collection of the same.