Don't minify CSS in development mode

For ease of debugging, this gives the option to turn off minifying CSS.

To generate expanded CSS, prepend the `rake` command with `env=development`. Examples:

    env=development rake generate
    env=development rake preview
    env=development rake deploy

Note that the first time doing this, you may need to run `rake clean` to force an overwrite of previously minified files in `public/stylesheets`.