pull latest zopfli as git submodule + Python 3 support

[anthrotype]

Hi,

Thank your for this project!

I'm interested in making the Python bindings up to date with zopfli's upstream repo, and also make it work on both Python 2 and 3, and on both unix and win32 platforms.

I would like to use zopfli instead of zlib for compressing WOFF 1.0 fonts in fonttools, a font manipulation library written in Python: fonttools/fonttools#491

Here I applied the following changes:

• I included google/zopfli repository as a git submodule. I think this is better than embedding copies of the source files. Whenever the upstream project is updated, you only need to checkout the latest commit from the submodule.
• I made some changes to allow the extension module to work on both Python 2 and 3, following the same approach as the other compression project by Google, Brotli -- to which I've also contributed.

Please let me know if this is ok.
Thanks,

Cosimo

[moyogo]

@adamdeprince ping?
It would be nice to have an updated version on pypi.

[adamdeprince]

My write access to this repo was terminated when I resigned from WNYC in 2014. I'll fork this and we can work from there.