Add US-specific demo site

This simple site illustrates how to use Oscar for a US-specific site.
It's intended to help developers build Oscar sites for the US.

It can be built using:

    $ make us_site

This will build the database and load some sample products.

The key changes from core Oscar are:

- Setting the default currency to USD in settings.
- Overriding Oscar's partner app so we can specify out own Selector
  class which returns the US strategy. This ensures all prices are shown
  without tax.
- Overriding the checkout session mixin so we can apply taxes to the
  submission and also pass a template variable that specifies taxes
  should be broken out separately. We use a dummy tax module that
  applies very simple taxes to the submission.
- Overriding the checkout shipping address form so we can force the
  state field to use the US state dropdown.
Document the new US sample site

- Rework the page on applying taxes in the US
- Update the release notes
- Update the "sandbox" page with details of the new site