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
CLN: move some functions from tools.merge to core.internals
CLN: calculate & use _ref_locs for unique items as well
Move get_series_dict to DataFrame
Use factorization when grouping ref_locs by block

Python3 complained about FloatBlock being non-comparable with NoneType
internals: import OrderedDict from compat

Fixes py26 tests.