Removed local-to-remote update scheduling.

The approach taken is incomplete; race conditions can happen.  Also, there needs to be a way to prevent fetching changes initiated by the app itself.
Added a check for listener types.

Only functions can be added as listeners.
Added syncing when onwrite is undefined.

Before this, onwrite augmentation only happened when FileWriter.onwrite was defined.

Also added a couple of clarifying comments.
Also removed the explicit `async = true` parameter from xhr.open calls.
Also changed file uploading to happen on fileReader.onload instead of onloadend.
Refactored token retrieval and saving.