IndexedDB: Add workaround for implementations that don't support autoIncrement.
IndexedDB: Add workaround for implementations that don't support autoIncrement.

Both Firefox and Webkit have issues with the autoIncrement feature of
IndexedDB.  Let's assume that those bugs will be fixed by the time the
spec becomes final and indexedDB is unprefixed -- but until that time,
don't use the feature and use the same this.uuid() key-generator that the
other implementations use.

brianleroux/lawnchair,brianleroux/lawnchair