Time scale improvements to improve performance and reliability

* Make parseTime private
* start on fixing time scale
* Reimplement existing functionality
* Tidy tests
* Fix labels for non-linearly sized units

Months, quarters and years have non-constant numbers of seconds. A scale that's linear WRT milliseconds produces incorrect tick labels due to the label formatting losing precision (eg year labels lose month and day so a label of 2016-12-32 displays as 2016 instead of 2017).

* Re-implement tick generation

As in v2.5

samples/scales/time/line-point-data.html

@@ -119,7 +119,8 @@
 
 		document.getElementById('addData').addEventListener('click', function() {
 			if (config.data.datasets.length > 0) {
-				var lastTime = myLine.scales['x-axis-0'].labelMoments[0].length ? myLine.scales['x-axis-0'].labelMoments[0][myLine.scales['x-axis-0'].labelMoments[0].length - 1] : moment();
+				var numTicks = myLine.scales['x-axis-0'].ticksAsTimestamps.length;
+				var lastTime = numTicks ? moment(myLine.scales['x-axis-0'].ticksAsTimestamps[numTicks - 1]) : moment();
 				var newTime = lastTime
 					.clone()
 					.add(1, 'day')