feat(Observable): add class with .on, .once, .off, .emit

This class should replace one from the "observable" package.
This one is better because it doesn't require to call super() in child
class constructor. Instead you can just extend from it.
It also uses more straight-forward Observable implementation
and checks argument types.

This class uses "underscore" package as dependency.
Add "underscore" to package.json

Also add basic tests to observable.test.litcoffee