PerformanceWarning: what is actually the problem I can change?
I get several PerformanceWarnings when I store my dataframe in a hdfstore:

```
C:\portabel\Python27\lib\site-packages\pandas\io\pytables.py:1788: PerformanceWarning: 
your performance may suffer as PyTables will pickle object types that it cannot map
directly to c-types [inferred_type->mixed,key->axis0]

  warnings.warn(ws, PerformanceWarning)
C:\portabel\Python27\lib\site-packages\pandas\io\pytables.py:1788: PerformanceWarning: 
your performance may suffer as PyTables will pickle object types that it cannot map
directly to c-types [inferred_type->mixed,key->block0_values]

  warnings.warn(ws, PerformanceWarning)
C:\portabel\Python27\lib\site-packages\pandas\io\pytables.py:1788: PerformanceWarning: 
your performance may suffer as PyTables will pickle object types that it cannot map
directly to c-types [inferred_type->unicode,key->block0_items]
```

What I can't get from this is what column gives me these problems, at least I don't have any "block0" columns :-) It would be nice if this warnings can give me an indicator what i can actually do about this warnings.
