Backport off to v2 as non-breaking change

[patrickroberts]

As a continuation of #58, I'd like to submit a request to backport off() to v2. This is because browserify does not want to upgrade to v3 due to the breaking changes that come with it.

I don't believe this should be difficult to implement, it just needs to be an alias of removeListener() which would (probably) just be a one line addition.

[Redmega]

I'd like to second this. if on is implemented off is the natural flip-side of that coin.