Add warning to link.js (#3572)

* Add warning to link.js

* Formatting changes.

* Move warning in link.js, make invariant.

* Update link.js, nit.

Author: lucasgodshalk

modules/Link.js

@@ -1,5 +1,6 @@
 import React from 'react'
 import warning from './routerWarning'
+import invariant from 'invariant'
 import { routerShape } from './PropTypes'
 
 const { bool, object, string, func, oneOfType } = React.PropTypes
@@ -73,6 +74,11 @@ const Link = React.createClass({
   },
 
   handleClick(event) {
+    invariant(
+      this.context.router,
+      '<Link>s rendered outside of a router context cannot handle clicks.'
+    )
+    
     let allowTransition = true
 
     if (this.props.onClick)