Removed code duplication for checking the bounds of column widths
- Added the opportunity to customize rendering of header cells an option "headerCellRenderer" can be specified as a functionwhich gets the current column definition. It can return anything than the .html() method of jquery can deal with.
- Added two options to customize header cell sizing. fitHeaderToContent makes the header cells be as wide as the content of the header cell. forceFitHeaderToContent restricts the minimal width of the header cells to the size of the header cells content.

Summarized thins change makes it possible that users take control over the rendering of header cells. Even if the width of the header cell needs to change.