fix bsTypeahead: selected object display

Fix for #1260:

bsTypeahead expect a label property on a selected obj when the ngOptions allows to set any attribute as label.

The fix uses $parseOptions.displayValue instead of the hardcoded `label` attribute.
Do we need to match, can't just use the modelValue?

```
      controller.$render = function () {
        controller.$modelValue, controller.$modelValue, 'controller.$viewValue', typeof controller.$viewValue, controller.$viewValue);
          if(controller.$isEmpty(controller.$viewValue)) return element.val('');
          var value = parsedOptions.displayValue(controller.$modelValue || controller.$viewValue);
          element.val(value ? value.toString().replace(/<(?:.|\n)*?>/gm, '').trim() : '');
        };
```