shows user's avatar if he or she is a hive's user

If the commit email is one of the hive's user's one, then shows the user's
avatar, otherwise shows the email's gravatar.
changed gravatar's default image to hive's default image
Merge pull request #21 from keesun/HIVE-32

커밋 히스토리에 보이는 아바타 이미지 수정