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/README.md

@@ -0,0 +1,29 @@
+# Animation Samples
+Sample apps that showcasing Flutter's animation features
+
+## Goals
+
+- Demonstrate the building blocks for animations and how they work together
+- Provide samples for common patterns and use-cases
+
+## Samples
+
+### Basics
+
+Building blocks and patterns
+
+**AnimationControllerDemo** Demonstrates how to use an `AnimationController`.
+
+### Misc
+
+Other uses-cases and examples
+
+**ExpandCardDemo** Demonstrates how to use `AnimatedCrossFade` to fade between
+two widgets and change the size.
+
+## Other Resources
+
+- [Introduction to animations](https://flutter.dev/docs/development/ui/animations)
+- [Animation widgets](https://flutter.dev/docs/development/ui/widgets/animation)
+- [Flutter cookbook - Animations](https://flutter.dev/docs/cookbook/animation)
+- [Animations tutorial](https://flutter.dev/docs/development/ui/animations/tutorial)