advanced interface (and some cleanup)

I've taken the liberty to provide an advanced interface to the Kuler API.

Instead of just fetching a single, random color theme, one might now use `kuler.fetch_theme` and use the `kuler.build_url` options to search for more themes.

I stuck to the old Ruby 1.8 syntax, though I only did test it against current MRI 1.9.3.