add support to parse `Do` #1475

```
moment('May 6th', 'MMMM Do').format('MMMM Do')
// before => May 1st
// after => May 6th
```

To assure that the patch is working as expected, a temporary
test file has been added. It can still be found at
https://gist.github.com/gr2m/3c56d1ff91bec49f547c