Merge pull request #3137 from jreback/GH3070

ENH: GH3070 allow string selection on a DataFrame with a datelike index, to have partial_string semantics (like Series)
Merge pull request #3124 from jreback/GH2817

BUG: GH2817 raise the correct KeyError that the multi-index is not sorted
BUG: GH2903, implemented xs for axis=1 with a level specified