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".

remove preinstall hook that compiles stuff, we will use precompiled binaries to make it easier for people

make libsass as subtree, in the future do "git pull -s subtree libsass master" to pull in new changes