Add the ScopedModel Counter sample (#41)

Adds sample app that mimics the beloved starter app, but with ScopedModel for state management.

Contains a smoke test and is set up to run it in travis_script.sh.

scoped_model_counter/test/widget_test.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:scoped_model/scoped_model.dart';
+import 'package:scoped_model_counter/main.dart';
+
+void main() {
+  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
+    // Build our app, provide it with a model, and trigger a frame.
+    await tester.pumpWidget(
+      ScopedModel(
+        model: Counter(),
+        child: MyApp(),
+      ),
+    );
+
+    // Verify that our counter starts at 0.
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsNothing);
+
+    // Tap the '+' icon and trigger a frame.
+    await tester.tap(find.byIcon(Icons.add));
+    await tester.pump();
+
+    // Verify that our counter has incremented.
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsOneWidget);
+  });
+}