Word

[dwhieb]

This PR adds a Word model, as well as Utterance.words. closes#154

• Improved a lot of the JSDoc comments so that the developer documentation is more readable.
• Added Model.defineValidatedProp, which defines a property on an object that has to be validated using a validation function. The validation function should throw an error if validation fails.
• Had to stop using most private class properties. It turns out the way I was initializing them in Object.defineProperties wasn't actually doing what I thought it was. Short story: You can't access private properties (#prop) using bracket notation (this[prop]). So I adjusted the static methods on Model to compensate.

[dwhieb]

[javascript] Word

[dwhieb]

@vadekh I need to merge this to use it in the app, but you're welcome to still review it and suggest changes.