Added columnClassFunc option to specify class selectors for column sizin...

...g instead of widths.

Added extraColumnClass for specifying extra column classes.

For example the following can be used if using a Grid Responsive layout system to make the layout revert to single column on mobile devices.

$("myContent").columnize({
	columns: 3,
	extraColumnClass: "s-grid-whole",
	columnClassFunc: function(col) {
		return "grid-" + (12/col);
	},
});