Easier extendibility of rpivotTable() for custom S3 classes

[vh-d]

I would like to extend rpivotTable() for a my custom class (derived from data.frame) where some parameters (such as dimension for rows and columns) are predefined and user does not need to specify them. Currently one has to write ugly wrappers such as rpivotTableFromMyClass() to do the trick.

It would be wonderful if we could just use S3 methods dispatch.