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.)