unittest discover is not available in Python 2.6. Revert to 7f29234754be0f7d9a248ac0ef045d9306858eac.
Updated comment
Added development requirements.
Updated six version.
Update requirements.txt
Merge pull request #192 from kurniliya/fix-tox-py26

Fix tox py26 env (closes #191)
Install cov-core when running tests in tox and Travis. Required by coverage plugin
Our style is PEP8
Make test also work on Windows that uses backslash as path delimiter