Strange axis labels when fixedStepSize is not a factor of min and max

[fewstera]

I'm trying to use the tickOptions in order to manually set the labels for my yAxis. If I try to use a fixedStepSize that is not a factor of min and max then you end up with strange looking labels.

This codepen explains it best http://codepen.io/anon/pen/JRORQy

I'd expect the labels to be [1, 3, 7, 9, 11, 13, 15, 17, 19, 21] but instead they're [1, 2, 4, 6, 8 ,10, 12, 14, 16, 20, 21]

[etimberg]

The tick marks are generated at https://github.com/chartjs/Chart.js/blob/master/src/core/core.ticks.js#L52-L84

It probably needs a special case for the options including min, max and increment. I'm happy to look at a PR for it