Add animation samples (#118)

* initialize animations sample project

* add two demos, set up routing

* clean up expand_card demo

* update README

* clean up expand_card demo

* dartfmt

* update comment

* address code review comments

* swap AnimatedContainer and AnimatedCrossFade

* use AnimatedSwitcher instead of AnimatedCrossFade

* Revert "use AnimatedSwitcher instead of AnimatedCrossFade"

This reverts commit e112e02549.

* rename expanded -> selected

* use  Dart 2.4.0 constraint

* update README

* address code review comments

update images
add curves to expand_card
update pubspec.lock

* add @override annotation

* add animations project to travis script

* add empty test for travis

* add copyright notice to animations/ project

animations/pubspec.yaml

@@ -0,0 +1,18 @@
+name: animations
+description: A new Flutter project.
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.4.0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+  cupertino_icons: ^0.1.2
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+flutter:
+  uses-material-design: true
+  assets:
+    - assets/