Simplify things by letting the server own an Express instance

Setting up and maintaining a Rendr app become more simple once the Rendr server can own its own Express instance.  We create this at `server.expressApp`.

The application that users Rendr can either use this Express instance directly (`server.expressApp.listen(port)`), or for more flexibility and the ability to mount multiple apps within the same codebase, this Express instance can be mounted on a separate Express instance, either at the root or at a path:

```js
app.use(server.expressApp);
```
or
```js
app.use('/my_cool_app', server.expressApp);
```