BUG: Fix to_datetime to properly deal with tz offsets #3944

Currently for certain formats of datetime strings, the tz offset will
just be ignored.
DOC: Clarify that DataFrame string formatters must return strings

I am not 100% sure that string formatters must really return *unicode* strings
(non-unicode strings seem to work, too), but I am leaving that because it's
what is already documentated.

Fixes #6052.
ENH: allow legend='reverse' in df.plot() GH6014
CLN: clear up some cython warnings
CLN: eliminate bootstrapping issue form generate_code.py
CLN: test_timeseries_legacy, specialize assert_

GH6175: replacing "assert_" with specialized form assertEqual. WIP: have not adjusted any instances of assert_( (... == ...).all() )
CLN: test_timeseries, specialize assert_

GH6175: replacing "assert_" with specialized form assertEqual. WIP: have not adjusted any instances of assert_( (... == ...).all() )
DOC: add links to other pages to contributing.rst
BLD: Cut Travis-CI build times by 30-50%
foo
Merge pull request #6218 from y-p/ironcache

BLD: Cut Travis-CI build times by 30-50%
Revert "foo"

This reverts commit 213593cc5e457d60f70786981d7522ff4a752b3c.
BLD: travis limit ccache to 60M
BLD: travis, skip cache upload when no changes
BLD: travis, reuse cython files if unchanged in last 5 master commits
Merge pull request #6220 from y-p/PR_fast_travis2

BLD: more fast travis
DOC: added references to argmin/argmin API changes to release notes/v0.13.0 relating to GH6214
TST: run clipboard tests explicitly
BUG/TST: groupby with mixed string/int grouper failing in python3 (GH6212)
Merge pull request #6222 from jreback/groupby_py3

BUG/TST: groupby with mixed string/int grouper failing in python3 (GH6212)