TST: add tests for index.groupby (GH5620)
BUG: Implement step in slice StringMethod

Resolves #8754.
Add test for __getitem__ syntax on str
Make step None (same as 1) for consistency
Merge pull request #8843 from jtratner/support-step-in-str-slice

BUG: Implement step in slice StringMethod
Merge pull request #8840 from jorisvandenbossche/gh5620

TST: add tests for index.groupby (GH5620)
BUG: Defined .size attribute across NDFrame objects to provide compat with numpy >= 1.9.1; buggy with np.array_split (GH8846)
Merge pull request #8847 from jreback/size

BUG: Defined .size attribute across NDFrame objects to provide compat with numpy 1.9.1
DOC: api updates for .size
BUG: fix negative step support for label-based slices

INT: make Index.slice_locs step-aware

BUG: fix PeriodIndex.searchsorted to accept Periods

INT: refactor time-related indices to use step-aware slice_locs

INT: refactor MultiIndex to use step-aware slice_locs

INT: enable second/microsecond partial string slicing