Options to format SendForm item list

Add SendForm options itemFormat & indexStart to allow list formatting.  

Ex: Can be used to format POST for simple asp.net mvc model binding:
```JavaScript
		simpleCart({
			checkout: {
				type: "SendForm",
				url: "/checkout",
				method: "POST",
				itemFormat: 'items[{index}].{name}',
				indexStart: 0
			}
		});
```
Results in post that will bind to list of items: 
http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx/
```
  items[0].name
  items[0].quantity
  items[0].price
  items[0].options
  items[1].name
  items[1].quantity
  items[1].price
  items[1].options
  ...etc
```

Example c# checkout model
```C#
	public class CheckoutModel
	{
		public string Currency { get; set; }
		public decimal Tax { get; set; }
		public decimal TaxRate { get; set; }
		public int ItemCount { get; set; }
		public List<CartItem> Items { get; set; }
	}

	public class CartItem
	{	
		public string Name { get; set; }
		public int Quantity { get; set; }
		public decimal Price { get; set; }
		public string Options { get; set; }
	}
```