TAJO-833
TAJO-833: NPE occurs when using the column as a alias name in the multiple DISTINCT. (Hyoungjun Kim via hyunsik)

Closes #11
TAJO-832: NPE occurs when Exception's message is null in Task. (Hyoungjun Kim via hyunsik)
TAJO-827: SUM() overflow in the case of INT4. (Hyoungjun Kim via hyunsik)
TAJO-808: Fix pre-commit build failure. (jinho)
TAJO-819: KillQuery does not work for running query on TajoWorker. (jaehwa)
TAJO-825: Datetime type refactoring. (Hyoungjun Kim via jihoon)
TAJO-825: Datetime type refactoring. (fixed missing changes)
TAJO-830: Some filter conditions with a SUBQUERY are removed by optimizer. (Hyoungjun Kim via hyunsik)

Closes #12
TAJO-834: Add Travis notification to issues@tajo.a.o and IRC. (hyunsik)