Ruler format event added

Added a ruler tick format event to allow custom formatting of scale ticks, so that application specific data can be reflected within the scale display itself.

I also added a unit test which validates the new code.

Example scale format handler:
```javascript
		scales: [{
			format: function(tick, startValue, endValue){
				if (startValue % (2 * 10)) {
					tick.addClass("formatterTestClass");
				}
			}
		}]