Added withSource parameter to TTCounterLabelDelegate

Added a withSource parameter to the TTCounterLabelDelegate. The intent of this is to specify the label causing the event when multiple TTCounterLabel objects are being used on a single view. TTCounterLabel.updateDisplay was also modified to send self as the withSource: parameter

I also modified the sample to demonstrate the use of the parameter; in this case the source is used to change the backgroundColor of the label based on the state of the counter.