Prefetch resources

[assaf]

Common usage for warmup is to fetch resources.

• prefetch.add(url, fetcher?) adds a resource with the URL and immediately starts loading it
• prefetch.get(url) waits for resource to load and returns it
• Fetcher function called with the URL, should return an object
• Default behavior: load resource, fail unless 200/304, parse if JSON
• Use with Google OAuth: prefetch.add(auth.google.prefetch)