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/android/settings.gradle

@@ -0,0 +1,15 @@
+include ':app'
+
+def flutterProjectRoot = rootProject.projectDir.parentFile.toPath()
+
+def plugins = new Properties()
+def pluginsFile = new File(flutterProjectRoot.toFile(), '.flutter-plugins')
+if (pluginsFile.exists()) {
+    pluginsFile.withReader('UTF-8') { reader -> plugins.load(reader) }
+}
+
+plugins.each { name, path ->
+    def pluginDirectory = flutterProjectRoot.resolve(path).resolve('android').toFile()
+    include ":$name"
+    project(":$name").projectDir = pluginDirectory
+}