Subquery support for GroupBy queries
Merge branch 'master' into subquery
Fix subquery merge issues
Fix NPE when subquery returns empty result
Documentation for query data sources
Merge branch 'master' into subquery
Merge branch 'master' into subquery
Fixed realtime querying after subquery changes
Merge branch 'master' into subquery
Extra null value check. Not sure why this is needed in the subquery implementation and not regular queries. Bug elsewhere?
Merge branch 'master' into subquery
Fixed realtime index query serving failure due to incorrect treatment of DataSource
Merge branch 'master' into subquery

Conflicts:
	processing/src/main/java/io/druid/query/BaseQuery.java
	processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java
	processing/src/test/java/io/druid/query/QueryRunnerTestHelper.java
	processing/src/test/java/io/druid/query/topn/TopNQueryRunnerTest.java
Cleaned up equals() and hashCode(), per code review
finalize variables and optimize imports
Fix DataSource serialization
Fix legacy data source serialization
Merge pull request #371 from N3TWORK/subquery

Subquery functionality for Group By queries