Add getPendingChanges method to RemoteFileManager.

All files in My Drive are scanned and treated as created files for the
first run. Subsequent runs will fetch changes with drive's changes.list
API.
The order of changes in Google Drive is significant.

Make pendingChanges an array instead of a dictionary that categorizes
changes by type.
Large changes to remote_files.js.

With RemoteFileManager.findPaths, it will be easy to associate remote
files with local files and it handles the special case where a file has
more than one parents.

Changes are persisted and parsed in a different way. Now pendingChanges
is a dictionary, where the key is file id and the value is updated
metadata. As a result, multiple changes to a single file are easily
consolidated and will be much easier to process.

Comment out findChildren, idChildrenMap for now. They're not used for
the time being.