Add FactoryGirl and DatabaseCleaner to Gemfile and do light config for specs
Refactor Season stats model

Season.batting_title_contenders -> Season.batting_title_eligible
Remove after_validation :associate_player -- will do this during seeding instead
Add core queries to Season stats model

Includes:
  - Season.batting_title_eligible
  - Season.league_batting_leaders
  - Season.triple_crown_winner
Add seeds files and import rake tasks
Add rake task for running the assignment