Use the App namespace instead of relative imports

Adding a `package.json` to the `App` directory allows us to import using the `App` namespace instead of relative paths like `../Containers/Foo`. Relative paths can break when refactoring which is why we prefer absolute paths.

App/App.js

@@ -1,5 +1,5 @@
 import React, { Component } from 'react'
-import {HomeScreen} from "./Containers/HomeScreen";
+import {HomeScreen} from "App/Containers/HomeScreen";
 
 export default class App extends Component {
   render() {

App/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "App"
+}

index.js

@@ -1,4 +1,4 @@
 import { AppRegistry } from 'react-native'
-import App from './App/App'
+import App from 'App/App'
 
 AppRegistry.registerComponent('Boilerplate', () => App)