Retrieving visualization table data

- `additionalParams` function updated to create visualization instance, prefs instance, store the spreadsheet data passed in additional params, and call `getData()` function
- `getData` function updated to update url value if passed in, call `getData` function of visualization passing appropriate options
- `onDataLoaded` function added to conditionally store the table data and call `showLayout` function or call `ready` function
- `ready` function added to make the gadgets rpc call of "rsevent_ready"