Fix an error when there is no buttons config

Merge pull request DataTables#118 from tfermm/master

Author: AllanJard

js/dataTables.buttons.js

@@ -48,6 +48,11 @@ var _dtButtons = DataTable.ext.buttons;
  */
 var Buttons = function( dt, config )
 {
+	// If there is no config set it to an empty array
+	if ( typeof( config ) === 'undefined' ) {
+		config = {};	
+	}
+	
 	// Allow a boolean true for defaults
 	if ( config === true ) {
 		config = {};