HDFStore unconvert string fix

https://github.com/pydata/pandas/pull/6166

cherry-pick these 2 (or you can just copy its not long)
https://github.com/jreback/pandas/commit/6c268de1be38a239e9a114aaf1723a85e44146f2
https://github.com/jreback/pandas/commit/33cb7fb8f1db54597e4e9b9653a78f611e954dac

and update your PR

This works on windows 3.3 now...see if it passes travis