v6.0.0 - Separate classes

[jfhbrook]

This rearchitects pyee to use a BaseEventEmitter base class that's fully synchronous, and adds the following subclasses for async use:

• AsyncIOEventEmitter - For asyncio coroutines. Similar to EventEmitter but doesn't work with Twisted.
• TwistedEventEmitter - For twisted coroutines and inlineCallbacks-decorated functions.
• ExecutorEventEmtiter - For threaded use with concurrent.futures or the futures polyfill.

It additionally reimplements EventEmitter on top of BaseEventEmitter with a deprecation warning.

One thing this doesn't solve for is interop between different types of event emitters, but I plan on punting on this until some later date.

[jfhbrook]

• [x] Add test for compat and twisted Failures on error
• [x] Write test to see if it's viable to wrap an emit in a try (should break the stack as in node but let's check)
• [x] Make error handling more consistent in asyncio and twisted EEs