Normalise configuration to match Compass configuration

Change all configuration options to match the Compass configuration reference, and add generated_images_dir option.

http://compass-style.org/help/tutorials/configuration-reference/

Instead of:
`css` there's `css_dir`
`sass` there's `sass_dir`
`image` there's `images_dir`
`relative` there's `relative_assets`
.. and so on.

This should help avoid confusion in the future. I updated the README.md to reflect these changes.