change property timeLabel from strong to weak to avoid reatin cycle.

[szupzj18]

I used this amazing label for my project, it's perfect for my requirements (😄 thanks, bro.) , but I found that the timeLabel property might have retain cycle issue.

- (UILabel*)timeLabel {
    if (_timeLabel == nil) {
        _timeLabel = self;
    }
    return _timeLabel;
}

when no specific UILabel object was passed into this object, it will make the timeLabel pointer point to itself ( which is strong assignment), cause the retain cycle.
I tested on my device and that is the case.

I make a demo view controller and add this label as a subview of it.
when I quit the demo view controller. The object still exist on mem graph, which means mem leak (caused by the ivar: _timeLabel)

if I make it weak property, it will dealloc properly.

(I re-opened this pr, because I forgot to create a new branch lol.)