ENH: support HDFStore in Python3 (via PyTables 3.0.0)
closes #3750

recently released PyTables 3.0.0 (and numexpr 2.1), which now support python3
were completely broken

These changed all data to be stored as bytes (with to/from encoding/decoding)

This PR supports an encoding argument (if you really want to encode your data),
and provides transparent access for python3 (and backwards compat) to 
even python2 written stores
