[CALCITE-493] Add EXTEND clause, for defining columns and their types at query/DML time
[CALCITE-478] Move CSV tutorial (Siva Narayanan)

Close apache/incubator-calcite#21

Also, the following pull-requests were merged long ago:

Close apache/incubator-calcite#4
Close apache/incubator-calcite#19
Close apache/incubator-calcite#22