Added BUCK to generated app with react-native-cli init

Summary:BUCK is faster than Gradle.
For example `gradle app:installDebug` vs `buck install app` is ~7 seconds vs ~2 seconds with warm caches.
This is just the beginning to allow people to become familiar with BUCK.
It is enough for running the app locally and testing on a device.

Gradle is still used for dependency resolution.
Closes https://github.com/facebook/react-native/pull/6733

Differential Revision: D3126328

Pulled By: bestander

fb-gh-sync-id: 56aad276036c029af7e0e23d60c46ba2f77b3d2c
fbshipit-source-id: 56aad276036c029af7e0e23d60c46ba2f77b3d2c