Add simple shopping cart

It is based on ReactiveCocoa bindings.

Here we have 3 main components - XIB, ViewController and Presenter.
XIB defines the UI, and hooks for ViewController (IBOutlets).
ViewController binds Presenter with these IBOutlets, and Presenter
actually implements the view logic:

* Calculate all prices when user enters any of box sizes
* Checks if name/address/zip fields are valid
* Checks if everything is valid so we can proceed

The view should respond to these accordingly:

* Update all the prices when user enters/changes any of box sizes
* Highlight name/address/zip fields by red is they are invalid
* Disable Next button and enable it only when everything is valid

ReactiveCocoa allows to do that in really declarative way, making
programming of all that stuff clean and simple. Impressing.

Video: http://cl.ly/0U3y1I0h1g44

Interesting parts: 
* Presenter: https://github.com/astashov/ReactiveCocoaExperiments/pull/1/files#L3R28
* ViewController: https://github.com/astashov/ReactiveCocoaExperiments/pull/1/files#L5R27