[driver:dropbox] use PDO:SQLite for cache of delta data
[driver:dropbox] add source comment
[driver:dropbox] fix _save() missing argument
[driver:dropbox] configurable PDO-DSN & DB table name
[driver:dropbox] error handling on checkDB()
[driver:dropbox] add options PDO_User, PDO_Pass, PDO_Options
[driver:dropbox] remove "?token_hash=..." from contentURL
[driver:dropbox] use `PDO::exec()` if possible