Add posts collection and post model; fetch post data from localStorage

My original plan was to create a base collection and base model class
and extend those to create the Posts/Post classes, but the properties
added with underscore 'extend' weren't appearing in the instance of the
child constructor.

As a short term solution, making one class for each gets the point
across just as well. Inheritance in JS is another whole mess and having
a data layer is a good start.

Also needed to save data as JSON and parse the JSON when saving it or
fetching it from localStorage.