ENH: Add `isin` method to DataFrame.

docs. to be rebased

ENH: Add isin method to DataFrame

Basic tests.

Added method and fixed tests.

ENH: Add ordered argument to df.isin()

Expects a sequence of arrays.

Updated release notes for df.isin()

CLN: cleanup

going to remove ordered argument.

Using a dict for ordered matching. Docs

BUG: fixed subselection length check issues.

Updated release notes for df.isin()

remove merge conflict note
ENH tweak DataFrame isin method
ENH add iloc argument to DataFrame isin
Merge pull request #4258 from hayd/dataframe_isin2

ENH: Dataframe isin2