.un alias for .removeListener in events module

[rifler]

.addListener -> .on
.removeListener -> .un

[cjihrig]

Related - #540, #1064

[mscdex]

-1. un looks awkward since it's a prefix and not a word by itself, and it looks too close to on.

[ChALkeR]

-1. Does anyone else use that? If not, what's the reason for including it?

[jasnell]

-1. appreciate the idea of being able to type fewer characters but just not seeing the value here, not to mention that un and on are quite visually similar that it would likely lead to confusion.

[estliberitas]

Maybe .off then like in EventEmitter2?

UPDATE: sorry, just read #540

[ronkorving]

-1. I'm with @estliberitas here. If you're gonna add an alias (which I for one don't need), .off would be a clearer name.

[jasnell]

@rifler ... thank you very much for the PR. Given the -1's, I'm going to close this.