add pushover service

[jantman]

This adds a service for the Pushover free push messaging service. It's a completely free push messaging service with native clients for Android and iOS, as well as a web client. They have an open API and client libraries for a bunch of languages, as well as plugins or integrations in a bunch of services (TravisCI supports a Pushover method for build notifications).

The code here is pretty simple and straightforward, and seems to work fine. The logo came directly from Pushover; they have a ~600px SVG logo which I just scaled up to 1024x1024 and then rendered for the png. The setBadge() is setting the message counts properly, and client.pushover.net does Web Notifications on new messages.