Merge branch 'feature/autoreduce' of https://github.com/danelkhen/c3 into danelkhen-feature/autoreduce

`getXValue` returns the index when x is specifically 0, because it's falsey. Fixed that by checking the specific case.
I have not updated c3.min.js.

Sometimes the data provided can't be used for <code>transformForArcLabel</code> (a stacked area chart where registered items came back as neither connected or disconnected. http://jsfiddle.net/U5g3h/). I find that the <code>startAngle</code> and <code>endAngle</code> values arrive as <code>NaN</code>. To fix this I am checking before assigning <code>x</code> and <code>y</code>.