Made sizeThatFits: & thus sizeToFit work

Before this change OLImageView would return CGSizeZero when calling sizeThatFits:

From [UIView's documentation](http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIView_Class/UIView/UIView.html#//apple_ref/occ/instm/UIView/sizeThatFits:) for sizeThatFits: "UIImageView object returns the size of the image it is currently displaying"