Required column (and other features)

Major new feature: is now possible to declare a column as ``required`` (all cells in those columns must be filled)

Other minor features:
 * is now possible to display a columns help using ``col_description`` (another, longer, text after column label)
 * images gif icon now behave an alpha channel
 * when pressing "Add new row" button, input focus is given to the first cell in the new row.