Added insertOrReplace option

Added `WebSQLStore.insertOrReplace` option. It defaults to `false` (the previous behavior), but if set to `true` it will always issue `INSERT OR REPLACE INTO ...` statements instead of `UPDATE` / `INSERT` statements.

This is helpful if the model changes are coming from another source (syncing with a server) and you want to avoid doing a `SELECT` against the database to determine if the record is already there or not.