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
TST/CI: remove html5lib from 3.2 build
Merge pull request #4278 from cpcloud/remove-html5lib-from-3.2-build

TST/CI: remove html5lib from 3.2 build
DOC: cookbook example
BUG: fix data.py regression
BUG: Fixed non-unique indexing memory allocation issue with .ix/.loc (GH4280)
Merge pull request #4283 from jreback/index_mem

BUG: Fixed non-unique indexing memory allocation issue with .ix/.loc (GH4280)
Merge pull request #4281 from cpcloud/data-py-regression-fix

BUG: fix data.py regression
TST: ujson dont force endianness #4274
Merge pull request #4284 from Komnomnomnom/ujson-tst-endianness

TST: ujson dont force endianness #4274
Update CONTRIBUTING.md with note on attribution in PRs
DOC: Fix typo.
Merge pull request #4285 from y-p/PR_CONTRIBUTING

Update CONTRIBUTING.md with note on attribution in PRs
Merge pull request #4286 from jseabold/fix-typo

DOC: Fix typo.
TST: print out byteorder in ci/print_versions.py
ENH: test_perf.py - use psutil to set affinity (if absent functionality - then affinity module)

original affinity module is not commonly installed, has known issues,
and was not updated since 2006.  psutil is already used in parts of
pandas, so it would be logical to use it instead by default.
minor: some trailing spaces and a pylint "pragma" to stop complaining about Series._ix defined elsewhere
ENH: ujson better handling of very large and very small numbers, throw ValueError for bad double_precision arg #4042