Add Pushover service integration

As requested [here](http://feedback.gitlab.com/forums/176466-general/suggestions/4849409-service-hooks-push-notifications-pushover-or-no) and [here](https://github.com/gitlabhq/gitlabhq/issues/1839), I propose an integration of Pushover service for project.

You can see more information about Pushover here: https://pushover.net/

Don't hesitate to share your thinking if I can improve this feature! :)

That feature introduce select field type for services options.
Same problem as here: https://github.com/gitlabhq/gitlabhq/pull/7931#discussion_r18270583