Pycmx

Added Python 3.7 to tests

Also some documentation and code style tweaks.

* Documentation
* An interface tweak: we no longer accept paths, but accept file handles, which is more conventional.
* Tests.

Also an example program: `edl2scenelist.py` for creating scene lists from EDLs.

In this version much of the event code has been rewritten to better reflect transitions and multiple-deck events in a way that's more formal.