Fix Tag-it preventing selection of auto-complete values via keyboard

If I configure a page with an standard ``<input>`` tag, the latest version of Tag-it and appropriate dependencies, if I try and do:

```
$('input[name="search_term"]').tagit({
    allowSpaces: true,
    caseSensitive: false,
    singleField: true,
    autocomplete: 
        {source: ['one', 'two', 'three'], //typically I use an external source here
        delay: 0,
        autoFocus: true}
});
```

then Tag-it works, except for when I'm attempting to select an item out of the autocomplete results via hitting tab or enter on the keyboard.  

``autoFocus`` is true, so the first item in the results is automatically selected when the autocomplete entries appear.  However, after entering some text (eg ``thr``), and hitting the Enter or Tab key, a tag is created with the ``thr`` text, rather than ``three``, which was the entry selected within the autocomplete menu.

This pull request ensures that during the ``keydown`` event, that a tag is only created if autocomplete isn't present/open, as the ``select`` handler for the autocomplete will handle creating the tag for us later.  Previously, the ``keydown`` event was prematurely creating a tag from the half-complete input & closing the autocomplete too early.