Added one more example use for conditional child

Example 2 conditional grandchild value at attribute
```javascript
#!/usr/local/bin/node

var libxmljs = require("libxmljs");
var xml =  '<?xml version="1.0" encoding="UTF-8"?>' +
           '<root>' +
               '<child foo="bar">' +
                   '<grandchild baz="fizbuzz">grandchild content</grandchild>' +
                   '<grandchild baz="fiz">grandchild content 2</grandchild>' +
                   '<grandchild baz="buzz" faz="3">grandchild content 3</grandchild>' +
               '</child>' +
               '<sibling>with content!</sibling>' +
           '</root>';

var xmlDoc = libxmljs.parseXml(xml);

var specificgchild = xmlDoc.get('/root/child/grandchild[@baz="buzz"]');
console.log( specificgchild.text() );
console.log( specificgchild.attr('faz').value() );
```