Squashed commit of the following:

commit 506fcbfa498b6e67febb8df41cb10d8a02a9519a
Merge: a0207d2 519a16a
Author: Hyunsik Choi <hyunsik@apache.org>
Date:   Fri Feb 14 01:58:00 2014 +0900

    Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/incubator-tajo into TAJO-305

commit a0207d295cff26c8c06cc7dd45a3976258426f59
Author: Hyunsik Choi <hyunsik@apache.org>
Date:   Mon Feb 10 19:17:58 2014 +0900

    killQuery works well. But, it requires refactoring.

commit d2952aafa684d6cccbe3c9d89d27e8882b7393b5
Author: Hyunsik Choi <hyunsik@apache.org>
Date:   Mon Feb 10 10:11:17 2014 +0900

    Improved subquery.

commit 1650a64520f6792aa428afc2e6aece5fdc71b6a2
Author: Hyunsik Choi <hyunsik@apache.org>
Date:   Mon Feb 10 02:08:39 2014 +0900

    added new state and event to Query.