Fix bug in loop in Document::emit_css when size_t is unsigned.

Cleaner integration with autotools

Also allowing files to be imported without an extension. I.e., users can say "@import 'foo'" when the file is named "foo.scss".

Allowing the Document constructor to read partials (i.e., the user can say "@import 'partial.scss';" when the file is named "_partial.scss".

Implemented the @warn directive. A bit glitchy if you try to pass it a list with arithmetic expressions and interpolants, but it generally works without barfing.