support for multiple classes (#124)

it is required when using react-jss with dynamic stylesheet - jss produces multiple class names then

src/CSSTransition.js

@@ -1,11 +1,14 @@
 import * as PropTypes from 'prop-types';
-import addClass from 'dom-helpers/class/addClass';
+import addOneClass from 'dom-helpers/class/addClass';
+
 import removeClass from 'dom-helpers/class/removeClass';
 import React from 'react';
 
 import Transition from './Transition';
 import { classNamesShape } from './utils/PropTypes';
 
+const addClass = (node, classes) => classes.forEach(c => addOneClass(node, c));
+
 const propTypes = {
   ...Transition.propTypes,