Hlookup

the following changes should fix the hlookup operator. I tested it on the latest Drill code, on a single node (embedded) with hbase and it's working fine.
Thinking about it, the user will always pass a string into table and column, so it will never actually contain a column from the subquery. This part can be made simpler, probably, but I will need more tests. Still, I will need to extract the strings here and pass them to lookup()