Change text on User#show page
Generate ChallengesController
Add route and definition for challenges#create
Add a button to challenge another user - issue #6
Add winner_id column to Challenge
Make the username a link to that user's page
Put all_types inside the Challenge class
Challenge someone using a button on their page

* Redirect to show the challenge afterwards
* Pass the user on the page as an argument to know who was challenged
* Define the show route