Upgrade node environment to Node.js 7.6.0+ (#151)

Upgrades the node environment to NodeJS 7.6.0.  Functions can now use async/await and promises; they can return a promise instead of using a callback.  The change preserves compatibility with the callback style.  

* Upgrade node environment to Node.js 7.6.0+

* Bump package version

* Preserve compatibility for callbacks

* Cleanup

* Re-add hello callback example

* Make sure not returning won't blow everything up

* Update stock example with request promise

examples/nodejs/stock.js

@@ -1,31 +1,37 @@
 'use strict';
 
-var http = require('http');
+const rp = require('request-promise-native');
 
-module.exports = function (context, callback) {    
-    let body = context.request.body;
-    console.log(`body text: ${body['text']}`);
+module.exports = async function (context) {
+    const body = context.request.body;
+    const symbol = body.symbol
 
-    var symbol = body['text'].split(' ')[1];
+    console.log(`Got symbol: ${symbol}`);
 
-    http.get({
-        host: 'finance.google.com',
-        path: `/finance/info?q=NYSE:${symbol}`
-    }, function(response) {
-        var resp = '';
-        response.on('data', function(d) {
-            resp += d;
-        });
-        response.on('end', function() {
-
-            try {
-                var parsed = JSON.parse(resp.slice(3));
-                var lastTrade = parsed[0]['l_cur']
-                callback(200, `{ "text": "${symbol} last traded at ${lastTrade}" }`);
-            } catch (e) {
-                callback(200, `{ "text": "Error (invalid NYSE symbol?)" }`);
+    if (!symbol) {
+        return {
+            status: 400,
+            body: {
+                text: 'You must provide a stock symbol.'
             }
-        });
-    });
+        };
+    }
 
+    try {
+        const response = await rp(`http://finance.google.com/finance/info?q=NYSE:${symbol}`);
+        const parsed = JSON.parse(response.slice(3));
+        const lastTrade = parsed[0]['l_cur'];
+        return {
+            status: 200,
+            body: {
+                text: `${symbol} last traded at ${lastTrade}`
+            }
+        };
+    } catch (e) {
+        console.error(e);
+        return {
+            status: 500,
+            body: e
+        };
+    }
 }