Added simple image slideshow viewer

This will float the images across the screen in popular screensaver
style, with Twitter messages superimposed across the bottom.

This now requires RubyGame and SDL. Installation instructions can be
found at https://github.com/rubygame/rubygame/wiki/Install.

A small bug in ruby-sdl-ffi on OSX requires the patch at
https://github.com/jacius/ruby-sdl-ffi/pull/4

Bumped version to 0.0.3