`master_image` for any platform

I added the ability to use the `master_image` parameter for any "allowed" platform.

Breakdown:
* Added a `is_supported_platform()` function which makes it easy to add qualified platforms.
* Separated the first `async.waterfall` argument in 3 parts, where the platforms are gathered from the `request.favicon_design` properties, and are each given their own callback in case a provided image needs to be base64 encoded.

Possible improvements:
* Detect not only `master_image.src`, but maybe also `master.url`, where the type needs to be set to "url".
* Maybe the way that the waterfall array is being put together? (might be slow?)
* Maybe things like variable/property names; I tried to stay close to what was already there.