Added basic auth support. See #10.

If user wants to use basic auth, the user must supply the following information
as part of configuration.

```python

      "auth": {
        "username": "username",
        "password": "password",
        "realm": "Restricted Area",
        "hostname": "example.org",
        "port": "https"
      },
```

We need to write config.xml before the start of the zap server
because we do not have API to configure authentication in ZAP
after it is started up.