Panel4D can not be saved to hdf using the .to_hdf call
```
import pandas as pd, numpy

print pd.__version__

with pd.get_store('test.h5') as store:
  df = pd.Panel4D(data=numpy.random.randn(2,3,4,5))
  store.append('test',df)
  print store

df.to_hdf('test2.h5','test')
```

Output:

```
0.12.0
<class 'pandas.io.pytables.HDFStore'>
File path: test.h5
/test            wide_table   (typ->appendable,nrows->240,ncols->2,indexers->[items,major_axis,minor_axis])
Traceback (most recent call last):
  File "pandas_hdf5_bug.py", line 10, in <module>
    df.to_hdf('test2.h5','test')
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/core/generic.py", line 521, in to_hdf
    return pytables.to_hdf(path_or_buf, key, self, **kwargs)
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 194, in to_hdf
    f(store)
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 190, in <lambda>
    f = lambda store: store.put(key, value, **kwargs)
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 585, in put
    self._write_to_group(key, value, table=table, append=append, **kwargs)
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 909, in _write_to_group
    encoding=encoding, **kwargs)
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 846, in _create_storer
    error('_STORER_MAP')
  File "/Users/kghose/Library/Python/2.7/lib/python/site-packages/pandas/io/pytables.py", line 815, in error
    (t,group,type(value),table,append,kwargs))
TypeError: cannot properly create the storer for: [_STORER_MAP] [group->/test (Group) '',value-><class 'pandas.core.panelnd.Panel4D'>,table->None,append->False,kwargs->{'encoding': None}]
```
