automated changes by python-modernize (without fix_unicode for now)
setup.py: require 'six', add classifiers for 2.6 and 2.7
manual py3 compat fixes
py3 compat: fix str/unicode for exception base class
py3 compat: fix base64_(en|de)code, int_to_bytes / bytes_to_int

also be more precise in docstrings about datatypes
do not use %s%s to concatenate bytestrings (doesn't work with py3), cleanup exception msgs
fix default salt bug in make_signer
py3 compat: manually fix unicode/bytes
Removed six and fixed support for Python 3 stdlib JSON.