Generate and set welcome controller as default

Generate a new welcome controller/view and set it as root in
config/routes.rb. The '/' now points to the welcome page.
Change 'Create player' button to 'Sign up'

Start updating the view of the program to account-based by making the
player equivalent of an account to the application. The 'Create Account'
is removed from the players#index view and now in the application view.
Update tests for players-related features

Update the tests to navigate to the players#index instead of the root.
Update documentation

Run rake doc:app to update the documentation