add pushover notification support

This adds support for notifications via [Pushover](https://pushover.net/), a push messaging service with clients for Android, iOS, and desktop (browser-based). As far as I know, it's the only free (no monthly cost at all, and mobile device clients are free) push messaging service that has an open API.

I think this would be a very helpful notification method for Travis jobs.

I've done my best to provide complete code; I searched the repository for 'campfire' (which uses a very similar API) and duplicated those addons, making the appropriate changes. I also have companion PRs for the travis-core and docs repos.

I've tested the pushover notification code itself - the part that POSTs to the API - and confirmed it working. However, aside from rspec tests, there doesn't seem to be any way to *really* test this. I'm very excited to get this feature, so I'd be more than happy to help with any testing - including setting up a repo on my GitHub account for testing this, and adding whoever as collaborators. I can confirm the pushover notifications, or I can setup a Pushover account with the free 5-day trial of their browser-based client, and pass credentials on to anyone who needs them.

In short, I *really* want this, and I'll do whatever it takes to help get the required testing done.

Thanks,
Jason Antman