add node test server to play with a mock backend

Can get this going by
```sh
cd testserver
npm install
# if you don't have supervisor:
sudo npm install -g supervisor
# supervisor watches files and reloads the server when anything changes
supervisor app.js
```
Then navigate to `http://localhost/9900` and see some data being loaded from the server through ActiveRest. At first you see `Loading...` then the app is re-rendered when data has come back.

I also modified `is_loading` slightly. This might not be the right fix but I want to be able to do:
```js
obj = fetch('/proposals');
if (is_loading(obj)) { ... }
```
which the prior version didn't support.