Doesn't work if ID exists in TEXTAREA tag

Ex: 
```
<label for="text">Message</label>
<textarea ui-tinymce ng-model="mymodel" name="text" id="text"></textarea>
```
                           

**Fixe**:
```
if (!attrs.id) {
    var uniqueId = uiTinymceService.getUniqueId();
    attrs.$set('id', uniqueId);
}
```