Provide a new customization option to solve issue #21 - when generating SASS files use image-url instead of url

Replaced legacy :csspath option with a new :cssurl option that allows customization of the full background url
attribute, **including** the "url()" part (e.g. not just the path)

Example, specify:

    :cssurl => "image-url('$IMAGE')"

or

    :cssurl => lambda {|image| "image-url('#{image}')" }

to generate styles that looks like:

    background: image-url('output.png') ...

instead of the default:

    background: url('output.png') ...

(see updated README for more details about using the new option)