Adding a first version of PushoverHandler

The PushoverHandler sends messages to api.pushover.net as proposed in  #111

The class depends on the SocketHandler
I modified the SocketHandler to make it possible to change the port for subclasses (Pushover needs the explicit port 80 to work).
The tests only see if the content sent to the api is correct. The rest should be already covered in the tests for the Sockets.