Add table.rebalance() and db.rebalance(), but more testing is still needed.
Merge branch 'reql_admin' into tim/rebalance-2981

Also fix some bugs at the same time, for no good reason.
Add files I forgot before.
Improve error messages in rebalance(); improve comments in ql2.proto
Merge branch 'tim/rebalance-2981' into reql_admin