ActiveRecord Associations 일부 번역

belongs_to 옵션 및 메소드 번역
ActiveRecord Associations 일부 번역

has_one 옵션 및 메소드 일부 번역
Merge branch 'master' of https://github.com/RORLabGuides/docrails
Translating association_basics.md
Merge pull request #38 from namsk/master

Translating routing.md : Rough Translation is done.
Translating association_basics.md
Translating association_basics.md

bug fix
Merge pull request #39 from seapy/master

ActiveRecord Associations 일부 번역부분