Add support for event namespaces

This will add support for namespaces on events known from jQuery (see http://css-tricks.com/namespaced-events-jquery/). My problem was that I was using the draggable plugin and I has problems registering a mousedown event because draggable also registered a mousedown event on the same element and unregistered it when I called fixed() on that element. My own event handler was removed, too. With namespaces, I can register and unregister "my own" events.

Example:
```javascript
rect.on('mousedown.myPlugin', function(event) {
     // do the work
});
```
```javascript
rect.off('mousedown.myPlugin');
```