Merge pull request #5593 from jreback/groupby_apply

BUG: Bug in groupby returning non-consistent types when user function returns a None, (GH5992)
BUG: reset setitem_copy on object enlargement
TST: eliminate SettingWithCopyWarnings in tests (catch them)
TST: tests for GH5597
TST: add testing base TestCase to provide global testing config
TST: changed refs to use testing/TestCase where setUpClass/tearDownClass are defined
TST: change from using unittest.TestCase directly to using pandas.util.testing.TestCase