[WIP] Uploader component

This is the beginning of wrapping the existing "uploader.js" here https://github.com/lholmquist/Ghost/blob/master/core/clientold/assets/lib/uploader.js

in something that the ember code can use.

it is mostly the same with some differences, such as exporting it as a module


i'm not sure if this should be a true "component" or just be imported into an existing component/controller
This seems to be an instance of where the code [differs from the original](https://github.com/TryGhost/Ghost/blob/master/core/clientold/assets/lib/uploader.js#L236), and was the source of a hard-to-find bug for me.