Add common convenient methods (such as -currentValue) + performance improvements

I've added a few common convenient methods, such as the ability to get label's current value at any given point of time (even in the process of animating). You can now set default animation duration as a property for the label instead of using 2 seconds or sending a magic number each time you want the label to animate.

Also, obvious CPU performance issue has been fixed — now if you ask a label to count to another value while it is still in the animating state, old timer gets invalidated before creating a new one.

P.S. Thank you for this handy class. 👍
The `NSTimer` holds a strong reference to the instance of the `UICountingLabel`, so we should change this to a `weak` reference to avoid a retain cycle and memory leak.