selectivity-input div uses same id when created from a "select"

```
<select id="bla">...</select>
...
<script>
$('#bla').selectivity();
</script>
```

Results in:

```
<select id="bla">...</select>
<div id="bla" data-name="bla" class="selectivity-input" ...>...</div>
```

No to elements in the DOM should have the same ID. A prefix or suffix of some sort should be used. E.g. chosen.js uses a suffix of `_chosen`, Select2 a prefix of `s2id_`
