"min_itemsize" doesn't work for MultiIndex columns in table format
```
df = pd.DataFrame([[1,2,3],[4,5,6]],
              columns=pd.MultiIndex.from_tuples([(1,'a'), (1,'b'), (2,'c')])).astype(str)
store = pd.HDFStore('/tmp/store.hdf')
store.append('test', df, min_itemsize={1 : 20})
```

yields

```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-cea3042a011b> in <module>()
----> 1 store.append('test', df, min_itemsize={1 : 20})

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in append(self, key, value, format, append, columns, dropna, **kwargs)
    915         kwargs = self._validate_format(format, kwargs)
    916         self._write_to_group(key, value, append=append, dropna=dropna,
--> 917                              **kwargs)
    918 
    919     def append_to_multiple(self, d, value, selector, data_columns=None,

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in _write_to_group(self, key, value, format, index, append, complib, encoding, **kwargs)
   1260 
   1261         # write the object
-> 1262         s.write(obj=value, append=append, complib=complib, **kwargs)
   1263 
   1264         if s.is_table and index:

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in write(self, obj, axes, append, complib, complevel, fletcher32, min_itemsize, chunksize, expectedrows, dropna, **kwargs)
   3783         self.create_axes(axes=axes, obj=obj, validate=append,
   3784                          min_itemsize=min_itemsize,
-> 3785                          **kwargs)
   3786 
   3787         for a in self.axes:

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in create_axes(self, axes, obj, validate, nan_rep, data_columns, min_itemsize, **kwargs)
   3466 
   3467         # validate our min_itemsize
-> 3468         self.validate_min_itemsize(min_itemsize)
   3469 
   3470         # validate our metadata

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in validate_min_itemsize(self, min_itemsize)
   3105                 raise ValueError(
   3106                     "min_itemsize has the key [%s] which is not an axis or "
-> 3107                     "data_column" % k)
   3108 
   3109     @property

ValueError: min_itemsize has the key [1] which is not an axis or data_column
```

... which is actually true (if "data_column" is interpreted as "queryable column" rather than just "column of data"), but should not be a blocker (at least, judging from the documentation).
