Add a POC OAuth client implementation

I've registered GitJournal as an app on Github and tried to get the
access_token within the app. This seems to work. Though, I need to
provide the client secret as well. It's quite sad that GitHub does not
even support the Implicit Grant auth type.

Credit: https://stackoverflow.com/questions/46196240/oauth2-flow-in-flutter-app

lib/oauthapp.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+import 'package:journal/apis/git.dart';
+import 'package:flutter/services.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+const _platform = const MethodChannel('gitjournal.io/git');
+
+// Actual message handler:
+Future _handleMessages(MethodCall call) async {
+  switch (call.method) {
+    case "onURL":
+      print("Call onURL" + call.arguments.toString());
+    // Do something nice using call.arguments["URL"]
+  }
+}
+
+class OAuthApp extends StatefulWidget {
+  @override
+  OAuthAppState createState() {
+    return new OAuthAppState();
+  }
+}
+
+class OAuthAppState extends State<OAuthApp> {
+  void initState() {
+    super.initState();
+    _platform.setMethodCallHandler(_handleMessages);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new MaterialApp(
+      title: 'OAuth App',
+      home: Scaffold(
+        appBar: AppBar(
+          title: Text('OAuth Test'),
+        ),
+        body: Column(children: <Widget>[
+          RaisedButton(
+            child: Text("Open OAuth URL"),
+            onPressed: () {
+              var url =
+                  "https://github.com/login/oauth/authorize?client_id=aa3072cbfb02b1db14ed&scope=repo";
+              launch(url);
+            },
+          ),
+        ]),
+      ),
+    );
+  }
+}