ENH: GH3070, extend slicing semantics for datelike indexed DataFrames with a string to work like
     TimeSeries (e.g. df['2001'] works)
DOC: cookbook examples
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)