TAJO-897: PartitionedTableRewriter is repeated several times with same table. (Hyoungjun Kim via hyunsik)

Closes #54
TAJO-898: Left outer join with union returns empty result. (Hyoungjun Kim via hyunsik)

Closes #55
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-902
added comments for table options