inital addition of an upload component

This is still a WIP upload component that is related to https://github.com/TryGhost/Ghost/issues/2547 and https://github.com/TryGhost/Ghost/issues/2426

basic usage in your `.hbs` file:

    {{gh-file-upload onUploadSuccess="onUploadSuccess" uploadButtonText="Save" autoUpload=false}}

When autoUpload is false, a button is shown.  the default is true

so far the exposed methods are

* `onUploadSuccess`
* `onUploadError`
* `onUploadComplete`
* `onUpload`

There should probably be a couple more, like a progress event, etc...


I just wanted to share what i had so far