Specifying no options throws error

As in some of the examples, using obj2xml with no options i.e.

```
var obj2xml = require('obj2xml');

var xml = obj2xml.convert({ example: 'text' });
```

Throws an error (specifically "Cannot read property 'disableMeta' of undefined").

Not a big deal to work-around by specifying empty options i.e.

```
var xml = obj2xml.convert({ example: 'text' }, {});
```
