WIP: add pandas.hdfstore suppport


includes some other commits to make testing clean
- https://github.com/jreback/into/commit/82a6546ee5c6465a851601873f00ff05109a3c0c is in another PR already
- https://github.com/jreback/into/commit/836d5caaf2e3189882a53df343a18a785672c3a2 should be cherry-picked

- need to solve the resource disambiguation issue

  'foo.h5' -> PyTables
  'foo.hdf5' -> h5py

  but these should be dynamically determinable via the inherent meta data in the file
This looks right to me.  Can I assume that `all(isinstance(df, DataFrame) for df in load())`?