ENH add date to DatetimeIndex
Allows the date to be pulled out from a DatetimeIndex easily/efficiently.

Similar to #3180, which was for the time-part to be extracted.

_From SO question: http://stackoverflow.com/questions/16563552/pandas-fancy-indexing-a-dataframe_.

~~I haven't added this to release notes, as maybe too late for 11.1?~~
