Basic compatibility with Python 2.6

- list extra requirements needed on Python 2.6
- provide extra requirements file for pip on Python 2.6
- use requirements-2.6.txt for Python 2.6 on Travis
- try using unittest2 if unittest module does not work

See issue #258
Do not use iterators for Python 2.6 compatibility

We should not have huge list anyway.

See issue #258