Unify account load event, default balance to null

Use ```Account.entry()``` instead of ```Account.on('entry', ...``` to unify the way we listen for account data.
This frees us from having to remove listeners, because ```Account.entry()``` initiates the request and only calls the callback once when the request finishes.
Initialize balance to ```null``` instead of ```0``` to be semantically correct.

Fixes #584.