added utility method to expand swipe animation

calling this from button callback function will animate its expansion if configured for it.

example use:
                                       MGSwipeButton * trashButton = [MGSwipeButton buttonWithTitle:@""
                                                                icon:[UIImage imageNamed:@"trash"]
                                                     backgroundColor:[UIColor colorWithRed:1.0f green:59/255.0f blue:50/255.0f alpha:1.0f]
                                                             padding:padding
                                                            callback:
                                       ^BOOL(MGSwipeTableCell *sender) {
                                           NSLog(@"Swipe delete");
                                           NSIndexPath * indexPath = [me.tableView indexPathForCell:sender];
                                           indexPath=indexPath;
                                           [me deleteMail:indexPath];
                                           [cell expandSwipe:MGSwipeDirectionRightToLeft animated:YES];
                                           return NO;
                                       }];