add @match attribute for Chrome

Without `@match` attribute, Chrome makes it look like the script could be accessing the user's data on all sites.

Without `@match`:
![rXcO00D](https://f.cloud.github.com/assets/1422263/136977/ea6df5e0-715d-11e2-8c32-ce8be3c2fa67.png)

With `@match`:
![j4RtsDY](https://f.cloud.github.com/assets/1422263/136978/ef93f4de-715d-11e2-9d4c-5b77bb0961d8.png)