pushover support

This adds basic support for Pushover (pushover.net) messages.  Right now it can:

* Send a pushover message to a mobile device whenever a player joins
* Avoid sending notifications for ignored players.

It uses the requests library because it is awesome, but urllib could be used if you want to cut down on dependencies.

Pushover supports delivery groups, so one could send messages to multiple users.  This can be useful when running a private server so that you know when other players are online.