Merge branch 'master' of https://github.com/michalkop93/c3 into michalkop93-master
Merge pull request #1 from masayuki0812/master

merging upstream
fixes errors on null data

fixes errors thrown when provided with null array e.g.
['data',null,null]
Call to `getXDomain` requires targets.

Error can be reproduced here:
http://jsfiddle.net/mikwat/bt8u9fh9/1/
resize the eventRect for single data points for tooltips to work
api: allow querying chart range
Fix svg height when gauge - #470
Introduce grid.lines.front option - #462
Cancel zerobased if axis.y.min/max specified - #454
Merge branch 'master' into tooltip-fix-single-data-point

Conflicts:
	c3.min.js
Add tooltip.show/hide API - #467 #509
Fix axis width when display:none on init - #468
Add axis.y.default and axis.y2.default option - #442
Fix data label on stacked line/area chart - #482
Fix data label position when brushed - #485
Merge branch 'master' of https://github.com/michalkop93/c3 into michalkop93-master
Fix conflict
Fix onclick on step chart - #489
Fix axis.x.tick.values for timeseries - #408
Changed .class properties to ['class'] to prevent IE8 JS errors, IE8 treats class as an identifier instead of a property