Changes for python3

[arnout]

Modern systems have /usr/bin/python pointing to python3 instead of python2, so breeder must work with python3 as well.

python3 requires bytes for base64 encoding and decoding and for zlib compression. To simplify things, open the file as binary for compressed or binary snake, text otherwise.

The result of base64 has to be converted back to text. This is different for python2 and python3, so use a version-dependent function d() for this. Since the result of base64 is pure ascii, we can use the ascii codec for this conversion.

breed_gtk_image wasn't tested but it looks OK.

[leobastiani]

could someone merge?

[BjarniRunar]

Haha, sorry about the absolutely ridiculous delay. Thanks for doing this!