Add chart.api ($$.api) to 'onunselected' callback

Patch for #1141

Update tooltip.js

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

It should behave as the 'onselected' callback.