New time scale ticks.mode/.source options

`ticks.source` (`'auto'`(default)|`'labels'`): `auto` generates "optimal" ticks based on min, max and a few more options (current `time` implementation`). `labels` generates ticks from the user given `data.labels` values (two additional trailing and leading ticks can be added if min and max are provided).

`ticks.mode` (`'series'`(default)|`'linear'`): `series` displays ticks at the same distance from each other, whatever the time value they represent, while `linear` displays them linearly in time: the distance between each tick represent the amount of time between their time values.

src/helpers/helpers.time.js

@@ -55,7 +55,7 @@ module.exports = function(Chart) {
 		var ticks = [];
 		if (options.maxTicks) {
 			var stepSize = options.stepSize;
-			var startTick = options.min !== undefined ? options.min : niceRange.min;
+			var startTick = options.min === undefined || options.min === null ? niceRange.min : options.min;
 			var majorUnit = options.majorUnit;
 			var majorUnitStart = majorUnit ? moment(startTick).add(1, majorUnit).startOf(majorUnit) : startTick;
 			var startRange = majorUnitStart.valueOf() - startTick;