Add localization and guiding tooltip for browser action

Hi Kelly,

once again, thanks for your excellent extension. I added some minor improvements I'd like to share. At first it wasn't quite obvious to me what the browser action is doing, so I added an explaining tooltip + localization for that.
It now shows *Add/Remove this site to/from the blacklist.* depending on the site status. Feel free to change the sentence if you think it's too harsh - I'm not an english native speaker, so my "feeling" for the language is not that great. The german translation actually reads *Rate this site as disturbing/harmless*  and resembles a friendlier style.

Thanks!
Max