Add source to Gemfile
Format syntax

Yes, consistency REALLY MATTERS!
Change to require_relative
Add tennis_game

Copy tennis_game_1 to tennis_game for refactoring. Also add case to test
suite.
Add Scoreboard to handle scores

A Scoreboard is responsible only for showing and counting scores for a
given player. It depends on a game (or anything that quacks like one).
Game should respond to a `#players` method.
Extract method ScoreBoard#reset_scores_count
Add Scoreboard#display to show the winner
Update Scoreboard to display "Advantage"
Update Scoreboard to display score labels
Update Scoreboard to display "Deuce"
Add Game#state. Remove logic from Scoreboard

Scoreboard should not know anything about the game logic to display its
result. Now it relies no the `TennisGame#state` method to decide what to
print.

The ouput is implemented in methods following the `#display_<state>`
naming convention.
Refactor Game#state with chain of states

Since the states names and game methods follow the same convention, we
can leverage from this and kill the `if` statements with a cleaner
solution.

In case the game does not have a winner, advantage or deuce, we fallback
to the default "show scores".