Added state check, confirmkeys attribute, and placeholder attribute to angular

Dirty / Pristine state tracked for ngModel attribute.
"confirmkeys" and "placeholder" attribute added to directive.

```
<bootstrap-tagsinput
              ng-model="cities"
              typeahead-source="queryCities"
              tagclass="getTagClass"
              itemvalue="value"
              itemtext="text">
              confirmkeys="13,188,9"
              placeholder="Add Tags"
</bootstrap-tagsinput>
```

Sorry for all the "changes" but my editor was setup for auto formatting with 4 spaces.