API: Get rid of the comparison operator.

Inspired by API changes in Python 3, 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.
Merge pull request #77 from peelonet/no-more-compare

API: Get rid of the comparison operator.