events: deprecate static listenerCount function

As per the discussion in #734, this patch deprecates the usage of
`EventEmitter.listenerCount` static function in the docs, and introduces
the `listenerCount` function in the prototype of `EventEmitter` itself.

PR-URL: https://github.com/nodejs/node/pull/2349


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>