Add DSN test utility

From the spec:

> Finally, provide a CLI to test your client’s configuration. Python example:
> 
> ```
> raven test http://public_key:secret_key@example.com/project-id
> ```
> 
> Ruby example:
> 
> ```
> rake raven:test http://public_key:secret_key@example.com/project-id
> ```
> 

Our usage:

```
cd example
go run test_dsn.go http://public_key:secret_key@example.com/project-id
```

This is not very fancy.