Generic 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