ruby_gtnp supports embedding image data directly in the notification
allowing remote notifications to display an image.
In practice, this change has been tested with Growl for Windows
receiving the notification and it behaves as expected, i.e. the image
appears in the notification.
This also likely resolves issue #231, as that involves Growl (for OSX)
refusing to download the images.
With Growl 2.0 being released, the images are shown properly
again, but they most be binary encoded and not a url as
described in the GNTP spec. This can be achieved by using
`127.0.0.1` instead of `localhost`, and ruby_gntp will send
the image as binary.
In addition, on GNTP registration Guard send its application
icon, so we have the nice Guard logo in the Growl preferences.