expr: multi-column join
expr: multi-column Join.on_left/right is list

Had to hack around args because list is not hashable
python: multi-column join
expr: multi-column join unique columns
sql: multi-column join
pandas: test multi_column_join
import data/meta into base
spark: into(SparkContext, Seq)
spark: ensure that curried get is used
fix spark __all__
skip merge test for serialization issues