Allow for custom placeholders

This change was originally authored by @bistoco

This is the safest way to allow users to specify their own custom placeholders, which is required when using tables.

Please merge

------ Original Commit ------

Added option for placeholder
- added custom markup for placeholder as option, useful when using with tables.
Example :
	// JS CODE
	$('table.data tbody').sortable(
	{  
		items: 'tr' ,
		placeholder : '<tr><td colspan="7">&nbsp;</td></tr>'
	}) ;

	// CSS STYLE FOR TR > TD
	.sortable-placeholder td
	{
		background:#FFFEB8;
		height:100px;
		width:100%;

	}