events: optimize adding and removing of listeners

events: optimize adding and removing of listeners

These optimizations result in >2x speedup in the ee-add-remove
benchmark:

* Don't mutate array.length when removing the last listener for
an event
* Don't bother checking max listeners if listeners isn't an array
* Don't call delete when removing the last event in _events, just
re-assign a new object instead

PR-URL: https://github.com/iojs/io.js/pull/785
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>