Support for named exchanges

Currently there is a single exchange created on boot and mounted at the root URL of the server. It'd be good to allow any number of named exchanges, creating many independent hubs for exchanging messages over. These could be setup at a "hubs" endpoint:

```
/hubs/my-exchange/
/hubs/my-other-exchange/
```

The test client and any future admin interfaces can then also be moved to more sensible URLs.
