Python3: __bool__, im_code, im_func
PEP-8 check all files, once in separate environment, parallel in Travis

Tox: PYTHONDONTWRITEBYTECODE = 1
python3 compatibility

- __next__ for iterator interface
- six.next() to get next item
- list(dict.keys())
- popen2.popen4 -> subprocess
- s2b -> b"..." literals
- deprecated assertEquals -> assertEqual
- hub_test test_fork using run_python
- 1L -> 1 long literal
- many PEP-8 fixes