added css style support for columns

see the following example

self.recordsViewModel = new ko.simpleGrid.viewModel({
        data: self.recordItems,
        columns: [
        	{
        		headerText: "Date",
        		rowText : "bookingDate",
        		cssStyle : ""
        	},
            { 
            	headerText: "Description", 
            	rowText: "desc", 
            	cssStyle : "" 
            },
            { 
            	headerText: "Category", 
            	rowText: function(i) { return getCategoryName(i.category); },
            	cssStyle : "" 
            },
           	{ 
           		headerText: "Value", 
           		rowText: function(i) { return "€ " + i.value; },
           		cssStyle : function(i) { return i.value > 0 ? "record-income" : "record-expense" } 
           	}
        ],
        pageSize: 25
    });