Introduce Button Component

Summary:
Button is an important component to help the community get onboarded with RN quickly, so the first few minutes of a developer's experience is not spent formatting a simple button component.

In my opinion, `<Button />` should be seen as a "lowest common demoniator" component, rather than "the one button to rule them all". In other words, we should only support features in Button that will work on any platform. We should encourage people to fork Button if they need to add specific features to it, rather than trying to twist and bloat this component until it supports everything.

These platform imitations may not have the perfect constants just yet, but they are good enough to make a user feel at home in the app, without any modification. The community can help tweak the final formatting to make them look just right- PRs are welcome!

Reviewed By: frantic

Differential Revision: D3929041

fbshipit-source-id: 3785fb67472a7614eeee0a9aef504c0bdf62ede7

Examples/UIExplorer/js/UIExplorerList.android.js

@@ -32,6 +32,10 @@ const ComponentExamples: Array<UIExplorerExample> = [
     key: 'ActivityIndicatorExample',
     module: require('./ActivityIndicatorExample'),
   },
+  {
+    key: 'ButtonExample',
+    module: require('./ButtonExample'),
+  },
   {
     key: 'ImageExample',
     module: require('./ImageExample'),