ENH: Address comments on github:
- Better docstrings
- Clarify oversample/undersample
- Inline the _circular_samples function
- Only shuffle if there are "unfair" class counts (otherwise it's a no-op)
- Fix various typos
- Rename all num_foo to n_foo.
- Add more doctests
- Simplify _fair_array_counts
- Use // instead of / for division.
ENH: Add more unit tests for preprocessing.