Here are some updates based on the latest round of python-dev feedback. It:
- changes the names of the named-tuple returned by wait()
- consistently uses the world "callable" rather than "function" or "method"
- clarifies the calling context of future callbacks
- removes the ability to remove call backs
- allows the same callback to be repeatedly added
- adds a pointer to pickle
- clarifies that the interpreter will not exit with running futures
- clarifies that module methods accept futures from different executors