HDFStore.select slowed by decode even when using columns=
I realized when profiling a slow select (200% more wall-time as direct pytables call and high memory usage)  that most of the time is spend inside bytes.decode called by [_unconvert_strings_array](https://github.com/pydata/pandas/blob/master/pandas/io/pytables.py#L3966), even when selecting only int64 columns. It seems spend time and memory to decode string that are never returned.

I'm using python 3.3 and latest pandas (commit 2d2e8b5146024a90001f96862cdd0172adb4d1b8).

I gladly get back with more details if needed.
