add Resources section
create basic prop4 text/graphic. still missing the angle graphic
add footer with CC0 info
Add justifications and notes for prop4. Also tweak Raphael canvas height in app.js
add description of proof-step clicking in intro
remove EleGroup code
move show/hide to Drawing and no longer pass in the raphael object to Point/Segment/Circle constructors
add capacity for drawing angles and use them in the prop4 graphic