Compatibility with Python 3 and pyScss 1.2.

- `implements()` doesn't work in Python 3
- Files are opened as UTF-8 by default, so reading from a PNG requires binary mode
- `string.lowercase` is no more
- `compress` has changed to `style` to match the Ruby implementation

Incidentally, if you don't have time to maintain this library, I'd be willing to do so; I did most of the work on pyScss 1.2 and use it with this lib for several projects.