Set map in 'add' and 'remove' event handler

See https://groups.google.com/d/msg/ol3-dev/R3pxYsM86Uk/ncoFTeId_GgJ

This allows
```
map.getOverlays().clear();
map.getOverlays().pop();
map.getOverlays().push(new ol.Overlay(...));
```
to work as expected