Adding initial Stripe example

Change-Id: I4ab9c7b106106eb632bffc488e75f6c7fbd2a65b

stripe/database.rules.json

@@ -0,0 +1,6 @@
+{
+  "rules": {
+    ".read": "auth != null",
+    ".write": "auth != null"
+  }
+}

stripe/firebase.json

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+{
+  "database": {
+    "rules": "database.rules.json"
+  }
+}

stripe/functions/index.js

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+const functions = require('firebase-functions'),
+      admin = require('firebase-admin');
+
+admin.initializeApp(functions.config().firebase);
+
+const stripe = require('stripe')(functions.config().stripe.token),
+      currency = functions.config().stripe.currency || 'USD';
+
+exports.createStripeCharge = functions.database.ref('/users/{userId}/charges/{id}').onWrite(event => {
+  const val = event.data.val();
+  if (val === null || val.id || val.error) return null;
+  return admin.database().ref(`stripe_customers/${event.params.userId}`).once('value').then(snapshot => {
+    return snapshot.val();
+  }).then(customer => {
+    const amount = val.amount;
+    const idempotency_key = event.params.id;
+    return stripe.charges.create({amount, currency, customer}, {idempotency_key});
+  }).then(response => {
+      return event.data.ref.set(response);
+    }, error => {
+      return event.data.ref.child('error').set(error.message);
+    }
+  );
+});
+
+exports.createStripeCustomer = functions.auth.user().onCreate(event => {
+  const data = event.data;
+  return stripe.customers.create({
+    email: data.email,
+  }).then(customer => {
+    return admin.database().ref(`/stripe_customers/${data.uid}`).set(customer.id);
+  });
+});
+
+exports.cleanupUser = functions.auth.user().onDelete(event => {
+  return admin.database().ref(`/stripe_customers/${event.data.uid}`).once('value').then(snapshot => {
+    return snapshot.val();
+  }).then(customer => {
+    return stripe.customers.del(customer);
+  }).then(() => {
+    return admin.database().ref(`/stripe_customers/${event.data.uid}`).remove();
+  });
+});

stripe/functions/package.json

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+{
+  "name": "functions",
+  "description": "Firebase Functions",
+  "dependencies": {
+    "firebase-admin": "^4.0.5",
+    "firebase-functions": "https://storage.googleapis.com/firebase-preview-drop/node/firebase-functions/firebase-functions-preview.latest.tar.gz",
+    "stripe": "^4.15.0"
+  }
+}