Loose frequency information when storing with table=True to HDF5
I have found what possibly is a bug.  When storing data as HDF5 with `table=True` the read value of series.index.freq from HDF5 is set to None rather than its original values.

Please see: http://stackoverflow.com/questions/16311045/in-pandas-why-is-frequency-information-lost-when-storing-in-hdf5-as-a-table

Not sure if there is a limitation for some reason in PyTables or if this is a bug.
