Add basic react-router@3 routing

Summary:
This commit introduces a `Page` component that we can use to provide
common styling and navigation to our pages.

wchargin-branch: use-rrv3

src/app/App.js

@@ -1,11 +1,19 @@
 // @flow
 
 import React from "react";
+import {IndexRoute, Route, Router, browserHistory} from "react-router";
 
+import Page from "./Page";
 import CredExplorer from "./credExplorer/App";
 
 export default class App extends React.Component<{}> {
   render() {
-    return <CredExplorer />;
+    return (
+      <Router history={browserHistory}>
+        <Route path="/" component={Page}>
+          <IndexRoute component={CredExplorer} />
+        </Route>
+      </Router>
+    );
   }
 }

src/app/Page.js

@@ -0,0 +1,13 @@
+// @flow
+
+import React, {type Node} from "react";
+
+export default class Page extends React.Component<{|+children: Node|}> {
+  render() {
+    return (
+      <div>
+        <main>{this.props.children}</main>
+      </div>
+    );
+  }
+}