Removed useless permissions

`clipboardRead` and `clipboardWrite` permissions don't seem to be used at all in the Text App. I didn't find any references to `execCommand` or to `clipboardData`.

`unlimitedStorage` permission is useful to get more 5 MB of local storage which is not what we need for now since the only thing we save are the settings.

http://developer.chrome.com/extensions/declare_permissions.html