Request: allow slice object in DataFrame.reindex
`irow` and `icol` allow slice.  Why not `reindex` too?

Note that not allowing slice object in `DataFrame.reindex` makes the following phrase in the document incorrect:

> Some might conclude that ix and reindex are 100% equivalent based on this. This is indeed true except in the case of integer indexing.

-- http://pandas.pydata.org/pandas-docs/dev/gotchas.html#reindex-versus-ix-gotchas
