Fix hardcoded sheet name in PivotTableCacheDefinition and allow other sheet to be the data source for the pivot table

In the Cache Definition there was a hardcoded sheet name of "Data Sheet", which is also in the example, which is why it worked. I fixed that by using the sheet name supplied by the pivot table.

I've also added an option to the pivot table options hash of :data_sheet which allows you to specify a separate sheet as the source for the table.

Maybe I should've put this request in 2 separate ones as I'm fixing a bug and adding a feature in one request....

Thanks for an amazing library!