BUG: HDFStore utf8 corrupted reads
When reading back utf8 encoded data, it randomly is corrupted when read back.
The output contains random values or fails with an `UnicodeDecodeError`.

After commenting out the fast version (`astype`) in `_unconvert_string_array` the issue did not show up anymore.

pyton 3.3, numpy 1.8.0, pytables 3.1.0

I'll try to get a reproducible test up in the next days.
