support rails 4 manifest.json format
- rails changed the way sprockets and the manifest file works in version
4, sprockets has been extracted to sprockets-rails, a plugin, and expects
manifest.json which has also changed to have a different meta-data format

{
	files: {
		"some-file-34984984984.js" : {
			"rootfile-OLDSHA.js" : {
          	"mtime": "2014-02-04T18:14:52.000Z"
          	"digest": "OLDSHA"
          	"size": 32
          	"logical_path": "rootfile.js"
        }
	}
}

- this caused some headaches, but nothing crazy to work around
notes about supported versions, changelog, rails 4
0.4.0