Fix #716: Allow column className to be a function

* A column's `className` property may now be a function or a string. If
  it's a function, it will be called when the row cells are created and
  passed the object being rendered in that row.
* Cells will always include the dgrid-generated "field-<field name>" class as
  long as the column has a field, even if a custom class is also specified.
Add Intern tests for column.className
CHANGES.md: Note support for column.className as function
ColumnSet: fix support for column.className as a function and add tests