sinon-chrome doesn't support promises for async functions

Hi,

Something along the lines of 
```js
browser.browserAction.getBadgeText({}).then(() => {})
```
will fail as getBadgeText return `undefined` instead of a promise (which is a supported way to work with the webExtension API).

As a side note, I tried to implement a fix for #94 but it fails due to properties ref and the code is a bit too obscure for me on that part haha.

Thanks !