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>
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
TEST io.sql: sqlite tests to seperate class

One base class for tests with sqlalchemy backend. So test classes
for mysql and postgresql don't have to overwrite tests that are
different for sqlite.
Merge pull request #6339 from cpcloud/replace-bool-fix

BUG: fix bool no-op replace calls
ENH: support per-axis, per-level indexing with loc[]

CLN: add comments in indexing code

CLN: comment out possibly stale kludge fix and wait for explosion

CLN: Mark if clause for handling of per-axis tuple indexing with loc

PERF: vectorize _spec_to_array_indices, for 3-4x speedup

PERF: remove no longer needed list conversion. 1.4x speedup
CLN: move indexing loc changes to index.py
TST: tests for per_axis_per_level_getitem
ENH: add core/indexing.py/_getitem_nested_tuple to handle the nested_tuple cases for partial multi-indexing