Merge pull request #4660 from jreback/hdf_empty

BUG: GH4273, appending a 0-len table to HDFStore was raising
BUG/API: (GH4584) to_hdf was raising when passing both arguments append and table
CLN: pep8 pandas/io/pytables
API: the ``fmt`` keyword now replaces the table keyword; allowed values are ``s|t``
     the same defaults as prior < 0.13.0 remain, e.g. ``put`` implies 's' (Storer) format
     and ``append`` imples 't' (Table) format