Redirect user after sign up

Redirect users after successfully signing up. Instead of the default
redirect to the root document, the users are now sent to "/players/new/"
to create a new player profile.
Create a method for testing sign_up process

Add a sign_up process to the authentication_helpers for testing the sign
up.
Switch the signing_up_spec to use the new method
Link player profile to a user

Add a reference to a user model in the player model.