coerce is multiply dispatched
spark coerce
spark test robust to spark absense
Skip gzip csv tests on windows py2.x
expr: Join automatically selects all shared columns
test arithmetic column operations

cc @talumbau
Merge pull request #372 from mwiebe/no_gzip_win

Skip gzip csv tests on windows py2.x
Merge pull request #373 from mrocklin/auto-join

expr: Join automatically selects all shared columns
Merge pull request #369 from mrocklin/coerce-rdd

Coerce works on Spark RDDs