Updated Icons to accept DivIcon & Icon classes

Hi,
I've updated the support for icons. Now the DivIcon and Icon class are supported. I close & unbind the popups because if the popupAnchor changes the popup has to be destroyed and recreated to modify its position. As there is no function to know if a popup is open then I don't reopen the popup.
While writing I just realized that this behavior can be improved checking for the change of the anchor. I will do it tomorrow.
The icons example has been updated.
Remember that Leaflet 0.6.4 has a bug in the unbind popup, so if the icon changes you have a 50% of chances that the popup will not open. This is fixed in Leaflet master, so it will work on 0.6.5 or 0.7 (depending on their decision).