Manifest v2

This adds the @"manifest_version": 2@ data to the manifest.json, which suppresses a warning in Chrome versions >18. See [the version documentation]("http://developer.chrome.com/extensions/manifestVersion.html") for a description. This covers issue #4, so I'll close that. 

The other two commits update the extension's own description in the manifest to be more specific, and add a line to the "wishlist" part of the Readme to have a smoother installation process.