Upgrade sklearn

Changes in this release:

• support for sklearn >=0.20
• fixes some deprecation warnings (e.g., for logsumexp)
• removes deprecated pyplot.hold() in /examples
• split out black dependency into dev-requirements.txt

Maintenance

• adds black auto-formatting ()
• fixes typo for variable inertia_
• fixes posterior_ when n_jobs != 1

Python3 Support

• cleans up requirements, tests, and examples to be python3 friendly
• normalizes inputs by default in both SphericalKMeans and MovMF clustering

Patch import paths

This release hopefully addresses #2

Inaugural public release.

First release to PyPI. Includes Spherical K-Means, Mixture of Von Mises clustering, and sample_vMF.