Add JSDoc to marionette.helpers

Used a bunch of descriptions from https://github.com/Puppets/marionette-docs/blob/master/api/helpers/helpers.md
I'm not sure if I would use the word `context` when describing a new Class. I usually hear the word context when you're talking about the value of `this` within a method. If you're looking to provide more information than 'create a new Class,' then perhaps something like Backbone's description of Collection's extend is preferable:

> To create a Collection class of your own, extend Backbone.Collection, providing instance properties, as well as optional classProperties to be attached directly to the collection's constructor function.