Migrate virtual-* dependencies to requirements file

They are only really required if you are developing Oscar's CSS files -
they are not a real dependency of Oscar.
Use CSS over Less as the default in the sandbox

You only want to compile Less files if you are developing the CSS of
Oscar.  Most people don't do this and so we default to serving
compressed CSS without Less compilation.  This also means that
the many contributors don't need to bother installing node.js and lessc,
which makes the installation process much simpler.

This change also re-introduces the `USE_LESS` setting which controls
whether Less files are used or not.
Update docs with how to develop CSS/LESS
Compile CSS before packaging in release script
Merge pull request #546 from tangentlabs/issue/546/less-node-css

Installation issues with virtual-node and virtual-less