A bug in HDFStore with MultiIndex
``` python
randn = np.random.randn
from pandas import *
index = MultiIndex.from_tuples([('a', 0), ('b', 'y')])
df = DataFrame(randn(3, 2), columns=index)
store = HDFStore('/tmp/testpandas.hdf5')
store['multi'] = df
```

raises `ValueError: invalid literal for long() with base 10: 'y'`
