[changed] Update to React 0.12. Fix warnings.

This PR makes react-modal require react 0.12 or latest. Also fixes the warnings and deprecation notices caused by the new API.

Also updated a dependency on react-tap-event-plugin package, since a newer, React 0.12 compatible version was published to npm.

The migration plan is the same published by the React team when migrating from older versions to React 0.12.