add imageOverlay events binding support by declaring clickable true.

see issue #2564, as @urban-1 points out, add the support to bind DOM events to imageOverlay. Although I did this in ```_initImage()``` function, the same as marker. A simple test fiddle can be seen at http://jsfiddle.net/francischan/qna75/3/ , I want to add some tests, could you please tell me where to put the test file? Or are there any existing, relevant tests I can build on ? (under debug/tests/ I think).