Add a keywords.txt file for syntax highlighting in the Arduino IDE

The Arduino IDE, in all its glory, isn't capable of recognizing our classes and functions.
So... We provide a glorious markup file!
(srsly, the arduino ide is crap. use miramole, visual studio/atmel studio, vim, emacs, nano and/or notepad++ to write your sketches!)
Merge branch 'master' of github.com:TacticalCode/SNESpaduino