Converted Extension persistent background page to event page.

From http://developer.chrome.com/extensions/event_pages.html,
*Event pages are very similar to background pages, with one important difference: event pages are loaded only when they are needed. When the event page is not actively doing something, it is unloaded, freeing memory and other system resources.*

It's pretty easy to do so. Let me know if it works for you.

![snp9upt](https://f.cloud.github.com/assets/634478/2120619/ea758c62-91bc-11e3-8488-6951ba2cf881.gif)