Backport a couple regression tests from `cytoolz`.

Also, add a test that ensures `curry` doesn't return a `partial`
object if a single argument remains and keyword arguments are present.
This behavior was added in #170.
Add serialization test for `complement`
Remove print statements in tests
Test groupby with lists of indices
Merge pull request #198 from eriknw/backport_tests

Backport a couple regression tests from `cytoolz`.