Character boxes

I added a readonly `characterBoxes` property to get the boxes that enclose each recognized character. `characterBoxes` is an NSDictionary where the keys are NSString encoded CGRects (through `NSStringFromCGRect`) and the values are the recognized characters (NSString). The keys can be converted back to CGRects with `CGRectFromNSString`. The CGRects are in UIKit's coordinate space (the origin is at the top left).