later 1.0.0

• Added private event loops: these are event loops that can be run independently from the global event loop. These are useful when you have code that schedules callbacks with later(), and you want to call run_now() block and wait for those callbacks to execute before continuing. Without private event loops, if you call run_now() to wait until a particular callback has finished, you might inadvertantly run other callbacks that were scheduled by other code. With private event loops, you can create a private loop, schedule a callback on it, then call run_now() on that loop until it executes, all without interfering with the global loop. (#84)