Allow Range to accept Infinites

I figured out that I don't need to deprecate the `__len__` method to make the `assert Range(0, oo)` test pass. I have to define an additional method which is `__nonzero__`[1] is python 2 and `__bool__` in python 3.

[1]: https://docs.python.org/2/reference/datamodel.html#object.__nonzero__
[2]: https://docs.python.org/3.4/reference/datamodel.html#object.__bool__