pd.HDFStore issues PerformanceWarnings for string columns
I get the following performance warning

> C:\Python27\lib\site-packages\pandas\io\pytables.py:1795: PerformanceWarning: 
> your performance may suffer as PyTables will pickle object types that it cannot map
> directly to c-types [inferred_type->mixed,key->block2_values]
> 
>  warnings.warn(ws, PerformanceWarning)

for the following DataFrame

> MSCI.ACWI           float64
> MarketCap           float64
> alpha               float64
> gics_code           float64
> investable             bool
> issuer_country       object
> universe               bool
> weight_benchmark    float64
> weight_portfolio    float64
> weight_active       float64
> dtype: object

The culprit is issuer_country column, which is a string column. I find it surprising. Is there a way to get around this?
