Can't use the same listener with different filters

In the events API you can't bind the same function callback to the same class with different filters. The actual behaviour is somewhat surprising:

```
from encore.events.api import EventManager, BaseEvent
em = EventManager()

def listener(event):
print event.source

em.connect(BaseEvent, listener, filter={'source': 1})
em.emit(BaseEvent(source=1)) # prints 1
em.emit(BaseEvent(source=2)) # doesn't fire
em.connect(BaseEvent, listener, filter={'source': 2})
em.emit(BaseEvent(source=1)) # doesn't fire
em.emit(BaseEvent(source=2)) # fires twice, printing 2 each time
```

I think it is reasonable to have the same callback bound with two different filters; however even if it isn't then binding the same function with a different filter should probably remove the old function.