Issue #11: Made huffman script Python 3 compatible

https://github.com/alobbs/libhpack/issues/11

The python script to generate the huffman tables was not version 3
compatible.

Now it should be fixed and should work on Python versions 2 and 3.

It has been tested in 2.7 and 3.3 versions.
Merge pull request #25 from Akrog/huff_gen

Issue #11: Made huffman script Python 3 compatible