Fix whitespace in README.md

Merge branch '1033_custom_legend_config' of https://github.com/davidpoore/c3 into davidpoore-1033_custom_legend_config

Calling resize on each and every window resize drammatically slows down the browser

Add a Gitter chat badge to README.md

Add title config option to c3 charts core

When all the data set is zero the label should be in the above of the y
axis

When all the data set is negative the label position of cero value will
be above the the y axis line then you will not see the label.

Set the X axis in subchart to visible or invisible

I'm totally new in the github and all this stuff. Sorry if I do something wrong. 

I came across c3 for two days and found it suitable for me. Thanks for that!

But there are some points which I would like to improve. Here I have added two lines to the c3.js
and with following options the X axis could be set to hide
```
...
axis: {
  x: {
    show: false
  }
}
...
```
http://jsfiddle.net/vladsu/89szjf3r/3/ 

Best wishes,
Vlad