Support `.encode()` keyword arguments for Python 2.6.
Use a generator expression instead to support Python 2.6.
Don't assume it's a named tuple to support Python 2.6.
Switch to `unittest2` to be able to use new features in Python 2.6.
Python 2.6 is now supported.
Update Travis CI configuration to also test with Python 2.6.
Update changelog to mention support for Python 2.6.
Be explicit with the import.
Correct dependency for Python 3.
Some clean up for Python 2.6 support.

* Removed unittest2 dependency on Python >2.6.
* Moved discover_tests.py into tests/.
* tokenizer.c: Fixed errors noted by -Wshorten-64-to-32.