Feature: Only show specific countries defined in options

We added a feature for showing only specific countries. By default, it still shows all countries, but if you specify `onlyCountries', it should only show the countries you put there. Also, if specified, preferred countries will only show countries from your 'onlyCountries' selection.

For example:

```javascript
$('#number_field').intlTelInput({
  onlyCountries: ['US', 'CA', 'PH']
});
```

Produces:

![screen shot 2013-10-09 at 12 59 43 pm](https://f.cloud.github.com/assets/46548/1295078/c7f847d4-309f-11e3-9a6e-678e596792af.png)