adds challenge exercises
changes competition_subscriptions and competition_exercises to be polymorphic
refactors competition subscriptions to registration and makes it polymorphic
fixes tests so they execute, adds computable exercises to challenges
fix registrations controller and competition registration
rename registration to competable_registration so that it doesn't conflict with devise
fixes  user registration associations
fixes public radio button for competitions
fix individual competition win
fixes team competition wins
fix competition page to hide/show limit text field based on win condition
sets up challenge types controller and finishes challenge form
fixes tests
Merge pull request #16 from madavik/polymorphic

Polymorphic