Fix issues in GSheetsTableWriter relating to asynchronous functions.
Fix issues in GSheetsTableWriter relating to asynchronous functions.

I didn't realize that the Google Sheets API's append function was asynchronous.
That caused problems when appending multiple rows in a short amount of time.
This fix resolves this issue. In addition, GSheetsTableWriter.append() now
has an optional callback parameter.

formulaslug/expense,formulaslug/expense