Since there is no demo or us site anymore we can simplify the structure
somewhat by moving the sandbox one directory up

The demo site was not kept up to date with the recent changes in Oscar
due to time contraints. It should be moved to a separate demo repository
in the future.

This might be resurrected in a separate repository in the future.

This reflects the change in the repo URL.

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

Several of Oscar dependencies don't support Windows, some of them don't
work at all. Added a warning that indicates installing Oscar on Windows
is a tricky process.

Fix a few factual errors and specify the correct URL for cloning Oscar.

In our instructions, it wasn't entirely clear that virtualenvwrapper is
required for setup.

Fixes #1088

Update release notes too.

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.