Kloppy

- Add metadata models (bdagnino)
- Move docs to https://kloppy.pysport.org/ (pratikthanki & koenvo)

- Fix to_pandas for Opta event data

- Fix in readme (rjtavares)
- Add additional_columns to to_pandas (rjtavares)

- Add Opta event serializer
- Fix for event pattern matching for nested captures
- Fix for event pattern matching when multiple paths can match
- Improved ball_recovery example

Add code formatting and contributing guide (dmallory42)
Add support for python 3.6

Fix Transformer when ball position is not set (benoitblanc)
Fix for working with periods in EPTS Serializer (bdagnino)