modify `cols()` and `rows()`

The `cols()` and `rows()` functions do not return nor can accept arrays.  A `d3.scale.ordinal()` with `.domain()` is needed to be pass.  

I thought it would be best to rename the functions, `colValues` and `rowValues`  and handle scale construction in the `_doRedraw` function.

Happy Holidays and let me know what you think.