Migrate to manifest v2

The version v.0.1.0 which is released on Chrome WebStore does not work on Chrome 27.0.1453.15 beta.
I got it work with this pull request applied.

Could you review it?

References:
* [Manifest Version - Google Chrome](http://developer.chrome.com/extensions/manifestVersion.html)
* [Tutorial: Migrate to Manifest V2 - Google Chrome](http://developer.chrome.com/extensions/tut_migration_to_manifest_v2.html)
* [Content Security Policy (CSP) - Google Chrome](https://developer.chrome.com/extensions/contentSecurityPolicy.html)