Add support to specify secrets in services

**What**
- During function creation, accept an array of strings defining swarm secrets
that are required for the service
- Update docs
- Add new guide on using the secrets capability
- Add new sample function to highlight using environment variables
- Update `ApiKeyProtected` sample function to utilize the new secrets
capabilities

**Why**
- This allows secrets to remain encrypted at rest instead of being unencrypted
in environment variables and yaml files.

Fixes #285

Signed-off-by: Lucas Roesler <lucas.roesler@gmail.com>

sample-functions/NodeHelloEnv/Dockerfile

@@ -0,0 +1,5 @@
+FROM functions/nodebase:alpine-6.9.1
+
+ENV NODE_ENV=dev
+
+COPY handler.js .

sample-functions/NodeHelloEnv/build.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker build --squash -t functions/nodehelloenv .

sample-functions/NodeHelloEnv/handler.js

@@ -0,0 +1,13 @@
+"use strict"
+
+let getStdin = require('get-stdin');
+
+let handle = (req) => {
+    console.log("Hello from a " + process.env.NODE_ENV + " machine")
+};
+
+getStdin().then(val => {
+   handle(val);
+}).catch(e => {
+   console.error(e.stack);
+});