Make less work with offline compression and document it
Commit CSS again (and use it by default in sandbox)

Oscar's CSS files had been removed from source control as they are
generated from the LESS files.  They were just dupliate content
ultimately.

HOWEVER, this was causing a few problems that warrant re-committing
them.

* To browse the sandbox, you were required to install lessc (and hence
  node.js) in order for the `COMPRESS_PRECOMPILERS` setting to work.
  This was too much of an overhead as most people playing with the
  sandbox just want to see how Oscar works and want to get it up and
  running as quickly and easily as possible.

* Oscar extensions also suffer from the same problem.  Eg, you would
  have to get lessc installed even if you had no interest in playing
  with Oscar's styles.

I think it's better to have the CSS files committed and simply to be
careful about pull requests and commits that change the CSS.  Before
releasing, we need to ensure that the CSS files correctly generate from
the LESS files.

This commit also changes the default sandbox set-up to serve the CSS
files but compressed.

Relevant to #605, #546, #246