Add click-out handler that helps closing the dropdown list while click outside the dropdown area

Based on @ghiden's comments, i did few changes on my solution.

1. Watch the `showDropdown` variable
    * Add a click-out handler for handling such a issue while `dropdown` list is displayed
    * Remove the click-out handler while `dropdown` list is removed from DOM
2. Remove the click-out handler while the `$scope` is destroyed