Update pykakao.py

buy() 인증 선택 가능(및 설명)
read() 변수 since 설명
테스트 결과 사용가능한 유저 에이전트까지 버전 입력
minor typo
added function ping()

PING support after this commit
cf. return result["body"] = {status}

This command is closely related with 'pingItv' keyword of the
result["body"] of the function buy()
minor typo

copy & paste error
minor type
Merge branch 'master' of https://github.com/su79eu7k/pykakao

Conflicts:
	pykakao.py