Unbind proxied events

Fixes #281
I can't agree more with you. The complexity/fragility comes from the attempt of patching (hacking) the status quo.
...and it can only get worse eg. if we decide to support namespaces for events.

If you want to introduce a new method to the API, I would suggest we go for `this.proxy` and "fool" people by saying that by default proxy events are disabled but you can 
```javascript
/* to enable the proxy */
this.proxy(true, otherArgsOrSettingsObject);

/* to disable the proxy again */
this.proxy(false, otherArgsOrSettingsObject);
```

or something else... I mean it feels odd to me that it is possible to proxy with `this.on()` but then have to use `this.offProxy()` to unbind.

Regarding the arguments the only way I can think of is passing an object with all the settings in it (just one argument) to `on` and `off`. If `arguments[0]` is not an object we fallback to the old `on/off` way of unpacking args or to the legacy methods that we could move to... legacy.js (along with this.defaultAttrs and other stuff and make a deprecation plan) - not cool I know. <- I realize that this could be very painful for twitter.com/tweetdeck and other 1, 2 guys ... I am just blabling :)