switch `chrome.runtime` for `chrome.extension`

So, after a little more research, I found there was an API change breaking tons of extensions. The `chrome.extension` API has been deprecated and removed in favor of the `chrome.runtime` API. Swapped those and it seems to be working now.

There are a few minor issues still:

When loading the extension, the following message is displayed:
        There were warnings when trying to install this extension:
            Unrecognized manifest key 'short_name'.

The menu is not very responsive - when a menu item is clicked, the item does not turn green. If you close and reopen the menu though it will highlight the appropriate item.

Closes #35