TAJO-850: OUTER JOIN does not properly handle a NULL. (Hyoungjun Kim via hyunsik)

Closes #22
TAJO-853: Refactoring FilterPushDown for OUTER JOIN. (Hyoungjun Kim via hyunsik)

Closes #28
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-884

Conflicts:
	tajo-core/src/main/java/org/apache/tajo/engine/planner/logical/join/JoinGraph.java
	tajo-core/src/test/java/org/apache/tajo/engine/query/TestJoinQuery.java
Fixed some unit test failures after rebase.