abstract away Manifest creation

[PoorBillyPilgrim]

ManifestFactory should feature methods that get/set all the appropriate info for each resource type for the Manifest, i.e. Canvas, Annotation, AnnotationPage, etc. The manifest factory should set a default for each resource's ids. It will accept the base URL (for now, it'll just be a relative path for now).

The factory should get to the point where all it needs is an ID URL for the manifest.json and a URL for the info.json file from the image server.