lychee.Input: prevent default behaviour only on valid bindings (was: Include event in triggered key events)

Consider the code below:
```
            this.__input.bind('down', this.__onKeyDown, this);
...
        __onKeyDown: function(position, event) {
            event.preventDefault();
            this.__player.goDown();
        },
```

That code is possible with this patch. Without this the screen would scroll down when I press the down key, which is not desired in this case.