SQL: add multi-index support to legacy mode (#6881)

and at once also index_label kwarg support
SQL: keep backwards compatibility for write_frame index handling
Merge pull request #6883 from jorisvandenbossche/sql-multiindex

SQL: add multi-index support to legacy mode
BUG: properly rename single group match in Series.str.extract()
Merge pull request #6888 from cpcloud/single-group-extract-match-naming

BUG: properly rename single group match in Series.str.extract()
CLN: replace pandas.compat.scipy.scoreatpercentile with numpy.percentile
CLN/TST: return Timestamp for .quantile on datetime[ns] series
Merge pull request #6810 from gdraps/replace-scoreatpercentile

CLN: replace pandas.compat.scipy.scoreatpercentile with numpy.percentile
DOC: note about query strings in HDFStore selection
Merge pull request #6905 from cpcloud/query-doc

DOC: note about query strings in HDFStore selection
API: update SQL functional api #6300
TST: test new read_sql api
Merge pull request #6867 from jorisvandenbossche/sql-api

API: update SQL functional api (GH6300)
BUG: TimeGrouper outputs different result by column order
Merge pull request #6908 from sinhrks/grouper

BUG: TimeGrouper outputs different result by column order
TST: restructure sql tests

- add some explanation of structure of tests
- general clean-up (some pep8, docstrings, etc)
- move some methods to _TestSQLAlchemy
- use standard quoting for sqlite
to_csv headers kwarg now works regardless of index flag
Add release note
Merge pull request #6916 from marcua/to-csv-headers

to_csv `headers` kwarg now works regardless of `index` kwarg
ENH: Implement Panel.pct_change

Had to change the implementation in generic.pct_change
to not pass `periods` as a keyword arg to generic.
Once the signitures of DataFrame.shift() and Panel.shift()
are aligned, this change can be reverted.