* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
Solution: update the documentation to reflect this. I've also removed
the documentation about the event plugin API. That part needs to be
refactored to work properly with BigchainDB 2.0. We can reintroduce it
later.
