XML entities aren't escaped

```javascript
require('twilio-js').TwiML.build(function(r) {
    r.gather(function(){}, {
        action: 'http://example.com/query?test=123&test2=456'
    });
})
```

This code produce incorrect XML because ampersand isn't escaped properly. So Twilio fails to parse it.

I included a quick fix for it, but the best solution probably will be to use a xml library.