Add warning for multiple children in Link (#558)

* Add warning for multiple children in Link

* Add space after if

* Use proptype validation

lib/link.js

-import React, { Component, Children } from 'react'
+import React, { Component, Children, PropTypes } from 'react'
 import Router from './router'
 
 export default class Link extends Component {
@@ -7,6 +7,13 @@ export default class Link extends Component {
     this.linkClicked = this.linkClicked.bind(this)
   }
 
+  static propTypes = {
+    children: PropTypes.oneOfType([
+      PropTypes.string,
+      PropTypes.element
+    ]).isRequired
+  }
+
   linkClicked (e) {
     if (e.target.nodeName === 'A' &&
       (e.metaKey || e.ctrlKey || e.shiftKey || (e.nativeEvent && e.nativeEvent.which === 2))) {