Add default constructor to SimpleXmlConverter

Thought I'd add this so that `SimpleXMLConverter` works as described in the website example --

```java
RestAdapter restAdapter = new RestAdapter.Builder()
    .setServer("https://api.soundcloud.com")
    .setConverter(new SimpleXMLConverter())
    .build();

SoundCloudService service = restAdapter.create(SoundCloudService.class);
```

Not sure how common of a use-case it would be to use the default `Serializer` implementation, `Persister`, without any customizations. Alternatively, I could just update those example.