Pull request for localization for Korean

Hi,

I added localization for Korean.

1. Korean resources
following files were added as Korean resources. I translated them based on Japanese version and modified some translations after using Korean version for some time.
locale/ko_KR/LC_MESSAGES/messages.mo
locale/ko_KR/LC_MESSAGES/messages.po

2. Adding Korean in the language list
I added Korean in the language list at functions.php
include/functions.php