Move directory-specific .gitignore to main dir

Move the `.gitignore` file from the `styles/` directory into the main
`.gitignore` file for visibility and ignore _all_ CSS stylesheets.
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).
Merge pull request #3 from alphagov/consolidate-gitignore

Move directory-specific .gitignore to main dir
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
Relax dependency on Sass

Relax dependency on the Sass gem to use any version from 3.2.x up to but
not including 4.0.

We use `Gemfile.lock` to specify known working versions so this should
not cause any issues.