fix multiindex xs slicing bug
BUG: GH3380 groupby will handle mutation on a DataFrame group's columns and fallback

     to non-fast apply

DOC: PTF
Merge pull request #3378 from cpcloud/multiindex-bug

multiindex xs slicing bug fix - thanks!
BUG: encoding detection should accept variations on 'ascii' GH3360
DOC: ref / val caveat, point at pandas methods

This in part addresses #3340.

I added a few comments in the doc that point users ad using the pandas
at, iat, loc, iloc, etc. methods and included an example similar to
the one exposed in #3340 that addresses some of the reference / value
intricaies encountered with pandas and numpy objects.

CLN: cleanup + edits - addresses recent discussion

CLN: cleanup II

CLN: wrap at 80 chars.

took care of both documents.
Merge pull request #3399 from dengemann/ref_value_data_access_doc

DOC: ref / val caveat, point at pandas methods - thanks!
Merge pull request #3384 from jreback/groupby_mutate

BUG: GH3380 groupby will handle mutation on a DataFrame group's columns
DOC: cookbook updates
     updates in indexing.rst for view vs copy
DOC: indexing.rst updates
TST: added testing and error messages for passing datetimes with timezones

     (not yet implemented, and error message was a little misleading)

CLN: better error messages on invalid appends
Merge pull request #3405 from jreback/hdf_dates

TST: added testing and error messages for passing datetimes with timezones
BUG: try harder to avoid ascii encoding GH3360

PTF
DOC: link to travis setup instructions in CONTRIBUTING.md
DOC: Its a bad idea to PEP8 documentation!
DOC: de-pep8ified indexing.rst and cleaned up the docs a bit
BUG: don't rely on sys.getdefaultencoding if we don't need to GH3360
ENH: save initial sys.getdefaultencoding() value aside for debugging
TST: ensure Period.strftime returns Unicode
BUG: Period.strftime should return unicode strings always GH3363
BUG: sort out unicode/str/repr and py2/3 for Period Objects GH3363

PTF