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)
TAJO-842: NULL handling in JDBC.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-842

Conflicts:
	tajo-common/src/main/java/org/apache/tajo/conf/TajoConf.java
TAJO-842: NULL handling in JDBC.