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

It's specified as a class attribute so any subclass can easily override
it. It's not used for WebSocket connections.

```
ChatExchange/0.dev (+https://github.com/Manishearth/ChatExchange)
```
Merge pull request #38 from jeremybanks/user-agent

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