Don't highlight properties named "off"/"on" as errors

Highlighting properties name `off` and `on` as errors is SUPER annoying when using [EventEmitter](http://nodejs.org/api/events.html)s since they have properties named `on` and [sometimes](https://github.com/hij1nx/EventEmitter2#emitteroffevent-listener) `off`.

Before

![screen shot 2013-08-25 at 12 23 13 am](https://f.cloud.github.com/assets/13998/1020828/a1043920-0cc9-11e3-99f3-270132f3d966.png)

After

![screen shot 2013-08-25 at 12 22 57 am](https://f.cloud.github.com/assets/13998/1020829/a55a9f8c-0cc9-11e3-90e0-f6a0ce81a614.png)