user: Treat names case-insensitively.
user: Treat names case-insensitively.

* Store user name case-sensitively.
* Do not allow case-insensitve duplication of user names.

This fixes #306.

bloodybear/yona,naver/yobi,violetag/demo,violetag/demo,oolso/yobi,Limseunghwan/oss,yona-projects/yona,ahb0327/yobi,ihoneymon/yobi,yona-projects/yona,bloodybear/yona,doortts/fork-yobi,bloodybear/yona,brainagenet/yobi,ahb0327/yobi,Limseunghwan/oss,bloodybear/yona,doortts/fork-yobi,doortts/forked-for-history,brainagenet/yobi,oolso/yobi,ihoneymon/yobi,naver/yobi,doortts/fork-yobi,ihoneymon/yobi,ChangsungKim/TestRepository01,ChangsungKim/TestRepository01,yona-projects/yona,brainagenet/yobi,ahb0327/yobi,oolso/yobi,doortts/fork-yobi,doortts/forked-for-history,doortts/forked-for-history,yona-projects/yona,Limseunghwan/oss,naver/yobi