Supersmoother

- Ensure compatibility with Python 3.6 and Numpy 1.12

- Fix installation issue [PR3](https://github.com/jakevdp/supersmoother/pull/3)

- Bug: assertion error was raised when period=0 was passed

New Features:

- Implement periodic smoothing

Bug Fixes:

- Fixed some issues with NaNs in corner cases of variable span smoothing

Bug fix release:

- Add tests and fix minor corner cases in utils
- By default, use fast rather than slow method for supersmoother

New Features:

- Implement bass enhancement
- Update example notebook with bass enhancement
- More complete unit test coverage

Bug Fixes:

- Algorithmic fix: previous version was not correctly interpolating in the last step of the supersmoother.