yobi-logo: add temoporary yobi logo for d2-fest

yobi 로고를 만들어 보았습니다.

![yobi-logo-temp01](https://f.cloud.github.com/assets/520523/867203/35cd1e7a-f6de-11e2-96de-99e7bdb1baa5.png)

![yobi-logo-temp02](https://f.cloud.github.com/assets/520523/867202/35841aa4-f6de-11e2-8322-ae38bf705a60.png)

앞 로고는 뭘 형상화 한 걸까요...? ^^;