Fixed additional class names issues

- Fixed cases where the colSpec was overwritten. This happened when
  combining the class names from the columnGroups with "cellClassNames"
- Added new columnGroups option "groupHeaderCellClassName" that is set
  on all the column group header (not only specific ones)
- Fixed logic that adds first/last group column class names to not
  overwrite the colSpec
Merge pull request #11 from PVince81/columngroupscellclassfix

Fixed additional class names issues