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/hello.js

@@ -1,4 +1,7 @@
 
-module.exports = function(context, callback) {
-    callback(200, "Hello, world!\n");
+module.exports = async function(context) {
+    return {
+        status: 200,
+        body: "Hello, world!\n"
+    };
 }