Fix season in index
Remove editing/deleting a player features.
Show players list as unordered list instead of table.
Fix score views/controllers about dealing seasons.
Add player/show
Fix redirection after creating a player.
Fix to pass the string to Model#order.
Add Score.top_scores.
Modify Score.top_scores to take difficulty.
Implement ranking in top page.
Add Machine's index page, and stub for individual page.
Fix links to the machine's individual page.
Add rankings for each machines.
Fix to Machine#musics_for.

Machine#musics_for returns nil when a season is not given and we aren't
in any seasons.