Network logos for Australian ABC, ABC2, ABC3 and Sky Arts

Added network logos ([screenshot](http://i.imgur.com/fjuij3Q.jpg)).
Note that according to Wikipedia, the ABC logo was changed in June, the provided one is current. Also, I couldn't find a light version to fit into the new dark theme, so I simply inverted the color.
This is my first time using Git and if I screwed up somehow, please let me know.
cc: @Supremicus