Add classes to wrapping div to allow better styling

Would be beneficial to add some descriptive classes on an ad's wrapping div (rather than just a numeric id) for better control via CSS. 

I think 'acm-wrapper' and the region name, such as 'acm-728x90_header' would be sufficient. 

A good use case is centering ad regions - currently you must match the id which means if the user ever changes regions, the CSS will need to be redone.
