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.
Fixes format

Unintentionally removed training spaces at the end of lines, which
messed up the formatting.
Highlights a new type of error

The assertion failures thrown by ck_assert_str_eq() were not being
highlighted.
Accept multi-word commands
Notify when child test processes crash
Merge pull request #25 from Akrog/huff_gen

Issue #11: Made huffman script Python 3 compatible
Fixes "make test"
Minor aesthetic change

Updates the formatting of the error reporting a crashing testing
process.