Finished basic Javascript API (#4)

Currently available:
- `nodecg.port`, `nodecg.host`
- `nodecg.sendMessage(messageName, messageData)`
- `nodecg.listenFor(messageName, function (data) { ... })`

Views have access to this object from HTML `<script>` tags, and from included .js files. 
Panels can only access it via .js files for the time being.