Set default executor as gevent since it works on both Python 2/3; both
the manager and transport packages allow user to pass in an executor
class at instantiation; may modify this user interface once I add
asyncio as an executor.

Shares essentially same interface as GeventExecutor

Passes all existing tests.  All concurrency and websocket code now uses
GeventExecutor class, which is injected (vs inherited like previous) by
user at Pubsub instantiation. I will modify AsyncioExecutor to
now utilize the same interface as the GeventExecutor.

Also refactored project layout a bit more.

Gevent has one additional test failing after refactor; need to
troubleshoot it before I release.

Change asyncio "join" method; construct "delayed_backgrd_task" method;
various other minor changes

This is versus the previous implementation of using static methods on
the executor and passing it explicitly to each one

Continue to work on getting transport server tests to pass for
asyncio.  There are two gevent transport server tests that need to
be fixed.  Also refactored out fakeredis, preferring to start
redis-server instance in test setup code

Several asyncio server class tests are the only tests still failing after
this major refactor. Focus on getting these to pass.

7 tests still fail