Added support for using a Seven Segment Display (SSD)

Initial support for a seven segment display. 

The code is based on an article I recently published over at <http://c7.se/7-segment-display/>

## Wiring example
![Wiring used in the example](http://c7.se/assets/7-segment-display/arduino_with_seven_segment_display.jpg)

## Note 
I also updated the RVM config to use `.ruby-version` instead of `.rvmrc`. (Not happy with having this in the repo. We should add a minimum Ruby version to the gemfile instead)