Zoom behavior change in v 0.4.10

The Zoom behavior seems to be changed after the fix made for #945

@masayuki0812 @aendrew I think the change for #945 has introduced issues in the zoom behavior

Check this [codepen](http://codepen.io/deepu105/pen/jBJKjP) which used version 0.4.9 and zoom behaves properly

Here is the same [codepen](http://codepen.io/deepu105/pen/gmEKNq) with version 0.4.10 which has a different zoom behaviour. IMO the behavior earlier was ideal.

I tried to set `axis.x.tick.fit = false` as per a stack overflow [comment](http://stackoverflow.com/questions/32691477/c3js-bar-width-not-adjusting-to-the-zooming) but that has a weird behavior as well check this [codepen](http://codepen.io/deepu105/pen/WpmKbb)

Reproduce the zoom difference using the linked CodePens for versions 0.4.9 and 0.4.10, then inspect the change associated with #945 and the behavior of axis.x.tick.fit. Done means restoring the earlier zoom behavior without the alternate tick.fit=false behavior shown in the third CodePen.

Written by the indexing model from the issue text.