Add JSDoc to marionette.helpers

Used a bunch of descriptions from https://github.com/Puppets/marionette-docs/blob/master/api/helpers/helpers.md
Technically, I think `this` is a new Object in here, right? The constructor function itself isn't related to the context?

`new MyConstructor() === MyConstructor.apply({});`?

Or maybe this is a common way of describing constructor function contexts and I'm just a noob.