TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects
DOC: docs to add detail on gotchas for true value testing
API/DOC: in HDFStore, change fmt -> format, change Storer format to Fixed format (just a name change);
         option for format is now fixed(f) or table(t)
API: add io.hdf.default_format option to enable an override to default storage formats in HDFStore
Merge pull request #4715 from jreback/hdf_format

API: change nomeclature in HDFStore to use format=fixed(f) | table(t)
Merge pull request #4657 from jreback/bool

API: GH4633, bool(obj) behavior, raise on __nonzero__ always
BUG: HDFStore.append_to_multiple - ensures rows are synchronized before writing

adds dropna kwarg + docstring + tests + documentation + release note

python3 compatible