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, even
  if a custom class is specified.
- Changes to the value of an editor column after row cells have been
  generated can be handled via a "dgrid-datachange" listener.