Pass the sort key when calling sort functions

Had a case where I needed to programmatically define my sorting functions and solved it with this patch. By passing the key into the sorting functions, you can write more flexible sort functions.

Here's an example of how this could be used. Assume you've got an unkown number of pages in the pages object.  

```javascript
var sortFunctions = {},
  sort = function( $ele, iso, key ) {
    return parseInt( $ele.find( '.' + key ).text(), 10 );
  };

for ( page in pages ) {
  sortFunctions['sort-' + pages[page].id] = sort;
}

$container.isotope( {
  getSortData: sortFunctions
} );

$( 'nav a' ).click( function() {
	var page = $( this ).data( 'page' );
	$container.isotope( { sortBy: 'sort-' + page.id } );
} );
```