Export deserializer

I have an API where russian text is encoded in base64, so I need custom deserializing method, which converts base64 to string like this:

```
var xmlrpc = require('xmlrpc');

client = xmlrpc.createClient({
  /* settings */
});

client.Deserializer.prototype.endBase64 = function(data) {
  var buffer = new Buffer(data, 'base64');
  this.push(String(buffer));
  this.value = false;
};
```

Exporting deserializer would probably help not only me.