Functional Enhancement: Responding to tapping event on current shown image

Add a optional delegate method for user.
`- (void)flowView:(PagedFlowView*)flowview didSelectPageAtIndex:(NSInteger)index;`

The idea is similar with `UITableViewDelegate method tableView:didSelectRowAtIndexPath:`

In many real cases, user may request do something implementation after tapping on the shown image.
The method allows user to do so.