Added label parameter for create_tag() function

This parameter could be used when label of the tag and the value of the tag are not the same.

Usage example:

$("selector").tagit({
    autocomplete: {
    	source: function( request, response ) {
		    $.ajax({
			url: '/url.json',
			dataType: "json",
			success: function( data ) {
			    response( $.map( data, function( item ) {
				return{
				    label: item.title,
				    value: item.id
				}
			    }));
			}
		    });
		},
		select: function(event, ui) {
			$("selector").tagit("createTag", ui.item.value, null, null, ui.item.label);
			// Preventing the tag input to be updated with the chosen value.
			return false;
		}
    }
});