Add Rakefile to render Sass and serve current dir

Add a Rakefile that renders the Sass stylesheet as CSS and serves the
current directory on http://localhost:9090 using WEBrick.

WEBrick code based on the `httpd()` method from Ruby's stdlib:
http://ruby-doc.org/stdlib-2.0.0/libdoc/un/rdoc/Object.html#method-i-httpd

(Unfortunately, I was unable to reuse that method apparently due to its
reliance on commandline-options, whereas I wanted to avoid shelling out
in the Rakefile.)
Use glob to compile all Sass stylesheets

Use `Dir#glob` in the Rakefile to catch all Sass stylesheets. This
avoids us hardcoding `gds.scss` (though the `styles` directory is still
hardcoded) and also means that `reset.css` will be rendered (it wasn't
previously).
Don't compile Sass partials

Don't compile partial Sass stylesheets, as a denoted by a leading
underscore in the filename. This matches Sass' intended behaviour:
http://sass-lang.com/guide#topic-4
Merge pull request #2 from alphagov/add-rakefile

Add Rakefile to render Sass and serve current dir