The `fields` parameter of the request has to include `nextPageToken`, and all request
parameters need to be present (in addition to the page token) in next-page requests.

The Google API isn't returning a `nextPageToken` for any of my test requests, though, so
I can't really test it yet.

Also tweaked formatting of Folder.getFile()

Moved all such code from Folder class into NetworkUtils. Still need to do the same for File.

This catches any errors that occur in code that starts a promise. It is currently used in
the init() function.

Also improved the warning message for gapi preload.

Now it caches the promise, unless an error occurs, in which case it clears that cached
promise.