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
spark: multi-column join
rename test to include spark to avoid testing
sql: import reduce from compatibility for py3
remove 3.4 testing.  replace when travis tests 3.4.1
Merge pull request #351 from mrocklin/multi-column-join

Multi column join