Don't crash on python -OO, where docstrings are None. Fix #88.

Add nose to tests_require so we can skip tests in python 2.6.
Standardize tests on pep_8_underscore_spelling, like the rest of the library.
Dodge spurious atexit error after running tests.
Switch to all the shorter nose assert helpers and the nose collector.

We'll be able to support non-class-based tests now.