API: Get rid of the comparison operator.

Inspired by [API changes in Python 3][1], I got rid of the comparison
operator and `__cmp__` magic method. You can achieve same functionality
with `__eq__` and `__lt__` methods, but with less clutter and
ackwardness.

[1]: https://docs.python.org/3/whatsnew/3.0.html#ordering-comparisons