Adds a User-Agent, and a test to ensure it's used.

There should probably be a default user-agent. This seemed to be a common pattern:

```
ChatExchange/0.dev (+https://github.com/Manishearth/ChatExchange)
```

It's specified as a class attribute on browser so any subclass can easily override it. It's not used for WebSocket connections.

[Successful Travis build here](https://travis-ci.org/jeremybanks/ChatExchange/builds/24064300)