RORLab 주간뉴스를 위한 Weeklynews 모델 추가

Mailchimp API를 이용해서 데이터를 최신으로 업데이트하는 기능도 추가
Weeklynews 컨트롤러 추가
Weeklynews 모델의 sened_at 인덱스 추가
주간뉴스를 주기적으로 수집하는 스케줄 추가
MAILCHIMP API 키 추가
주간뉴스 수집관련 코드 추가
gitignore에 맥용 찌꺼기 추가
주간뉴스 페이지에 구독하기 버튼 추가 및 버튼크기 변경
메인페이지에 RORLab 주간뉴스 패널 추가
Merge branch 'master' of https://github.com/RORLabNew/rorla

Conflicts:
	app/views/welcome/index.html.erb
	config/routes.rb
	lib/clock.rb