Remove stopRendering.js.

By [the how and why](http://piro.sakura.ne.jp/latest/blosxom/mozilla/xul/2009-12-18_stop-rendering.htm), we could be able to stop to use `stopRendering` because the old jetpack mechanism has been obsoleted.
Merge pull request #64 from saneyuki/stop_rendering

Remove stopRendering.js.

It was introduced to hide ugly visual glitch while processing multiple tabs. However, now Firefox works more quickly, and animation effects seems to decrease stresses from such visual glitch for me. Moreover, I've confirmed that the library doesn't work as expected on Firefox 26 at Ubuntu 12.04.