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.
DOC: clean-up docstrings of sql
DOC: fix sql docs (write example data to database)
clarified to_excel and read_excel

as discussed in issue GH6260
. io.rst: added a paragraph explaining the difference in behaviour from
0.12
. excel.py: added docstring for has_index_names
thanks to jmcnamara and jorisvandenbossche
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.
BLD: import error on py2.6 for print_versions
Merge pull request #6302 from iled/doc

DOC: clarified to_excel and read_excel (GH6260)
Merge pull request #6298 from jorisvandenbossche/sql-docstring

DOC: clean-up docstrings of sql + fix doc build
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