Fix Ruport::Data#Table method so it is class-level

In Ruport v1.6.3, `Ruport::Data#Table` is a class-level method.  In the current source it is an instant-level method, which caused my project to break (because we were invoking the class-level method) when I updated.

I believe that this got broken in 86f439fc when @Odaeus rightfully moved the location of the method from `Kernel` to `Ruport`, but didn't remember to set it as a class-level method.