Add support for hi resolution icons in the LR Chrome extension

Modifies the `setIcon` function to include paths to both the 19px icon and the 38px icon as described here [https://developer.chrome.com/extensions/browserAction.html#method-setIcon](https://developer.chrome.com/extensions/browserAction.html#method-setIcon) because Chrome extensions don't automagically pick up `@2x` images.

Using the icons from #21 works too!