Added base d20, rules, floating text

I've added some (very basic) rulesets for combat based on d20 games--the
six familiar ability scores (strength, etc), their modifiers--default to
10. Along with Reflex, Fortitude, Will, and Defense stats. Also added
basic attacks (just bump an entity to attack them) and floating sprite
text (the amount of damage you did--or if it was a miss--or if it was a
critical, which defaults to 20 damage right now--will 'float' above the
sprite, darkening and disappearing as it does).