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: Import testing into main namespace.
TST: run clipboard tests explicitly
Update groupby.rst
Update groupby.rst

Edit following this remark <https://github.com/pydata/pandas/pull/6304/files#r9564092>
CLN: Convert last of assert_(x == y) to assertEqual

Work on #6175. Fixes last of vanilla assert_(x == y) conversions in pandas/tests and pandas/*/tests. Tests still pass locally.
CLN: Convert assert_(x != y) to assertNotEqual

Work on #6175. Converts instances in pandas/tests and pandas/*/tests. Tests still pass locally.
Added assert_numpy_array_equals to make assert statments in this
be easy to understand.
Merge pull request #6315 from KTAtkinson/issue_6175

BUG: #6175 Got rid of assert_ statements in test_algos.py
Merge pull request #6308 from bwignall/assert_6175_7

CLN: Convert last of assert_(x == y) to assertEqual
BUG: Regression in join of non_unique_indexes (GH6329)
Merge pull request #6330 from jreback/align

BUG: Regression in join of non_unique_indexes (GH6329)
BUG: Issue with groupby agg with a single function and a a mixed-type frame (GH6337)
Merge pull request #6338 from jreback/agg

BUG: Issue with groupby agg with a single function and a a mixed-type frame (GH6337)
BLD: remove -v from nosetests for shorter output
BUG: fix bool no-op replace calls
CLN: replace in bool block rather than special-casing in common
TST: add tests for dict with bool keys
DOC: release notes
TEST: add basic postgresql tests

TEST: add postgresql to travis