feat(ticks): Add legend support

[ValentinH]

This feature enables to display legend under each slider tick.

A new getLegend option has been added that works pretty much like the translate function:

getLegend: function(value, sliderId) {
    if (value !== 0)
        return 'legend' + value; // will display 'legendX` under each tick
   return null; // won't display any legend
}

Also, now stepsArray handles objects like {value: 12} or {value: 1, legend: 'Legend for 1'}.

In order to get enough space to display legends under the slider, you need to add the with-legend class to the slider component. The default margin-bottom is then 40px which is enough for legend that are displayed on 2 lines. If you need more, simply override the style for the class.

Demo: http://jsfiddle.net/45y4hmzt/