python 3: print fixes
python 3: Introduce dependency on six and fix bytes / str issues
Merge pull request #13 from paultag/bugfix/python3

Bugfix/python3
add more python targets for travis
drop 2.6 from travis (we need OrderedDict)
pypy: fix CustomFields to use OrderedDict in to_json
Merge pull request #15 from paultag/bugfix/pypy-ordered-dict

pypy: fix CustomFields to use OrderedDict in to_json