Mixing tables.create_table and HDFStore.append
for example:

```
import tables as tb
import pandas as pd

class TestTable(tb.IsDescription):
    field1 = tb.UInt32Col()
    field2 = tb.UInt32Col()
    field3 = tb.UInt32Col()

h5file = tb.open_file('test.h5', mode = "w", title = "Test File")
table = h5file.create_table('/', 'test_table', TestTable, "Test Table")
h5file.close()

store = pd.HDFStore(h5file.filename, mode='r+', complib='zlib', complevel=5)

data = [{'field1': 1, 'field2': 2, 'field3': 3}, 
        {'field1': 2, 'field2': 3, 'field3': 4}, 
        {'field1': 5, 'field2': 6, 'field3': 7}]
df = pd.DataFrame(data)

store.append('test_table', df, mode='r+', append=True, format='table')
```

This code gave me an error:

```

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-5b74794d5ad5> in <module>()
----> 1 store.append('test_table', df, mode='r+', append=True, format='table', data_columns=['field1','field2','field3'])

/usr/local/lib/python3.4/dist-packages/pandas/io/pytables.py in append(self, key, value, format, append, columns, dropna, **kwargs)
    910         kwargs = self._validate_format(format, kwargs)
    911         self._write_to_group(key, value, append=append, dropna=dropna,
--> 912                              **kwargs)
    913 
    914     def append_to_multiple(self, d, value, selector, data_columns=None,

/usr/local/lib/python3.4/dist-packages/pandas/io/pytables.py in _write_to_group(self, key, value, format, index, append, complib, encoding, **kwargs)
   1266 
   1267         # write the object
-> 1268         s.write(obj=value, append=append, complib=complib, **kwargs)
   1269 
   1270         if s.is_table and index:

/usr/local/lib/python3.4/dist-packages/pandas/io/pytables.py in write(self, obj, axes, append, complib, complevel, fletcher32, min_itemsize, chunksize, expectedrows, dropna, **kwargs)
   3790 
   3791             # create the table
-> 3792             table = self._handle.create_table(self.group, **options)
   3793 
   3794         else:

/usr/local/lib/python3.4/dist-packages/tables/file.py in create_table(self, where, name, description, title, filters, expectedrows, chunkshape, byteorder, createparents, obj)
   1058                       description=description, title=title,
   1059                       filters=filters, expectedrows=expectedrows,
-> 1060                       chunkshape=chunkshape, byteorder=byteorder)
   1061 
   1062         if obj is not None:

/usr/local/lib/python3.4/dist-packages/tables/table.py in __init__(self, parentnode, name, description, title, filters, expectedrows, chunkshape, byteorder, _log)
    869 
    870         super(Table, self).__init__(parentnode, name, new, filters,
--> 871                                     byteorder, _log)
    872 
    873     def _g_post_init_hook(self):

/usr/local/lib/python3.4/dist-packages/tables/leaf.py in __init__(self, parentnode, name, new, filters, byteorder, _log)
    260         # is a lazy property that automatically handles their loading.
    261 
--> 262         super(Leaf, self).__init__(parentnode, name, _log)
    263 
    264     def __len__(self):

/usr/local/lib/python3.4/dist-packages/tables/node.py in __init__(self, parentnode, name, _log)
    244 
    245         # Is the parent node a group?  Is it open?
--> 246         self._g_check_group(parentnode)
    247         parentnode._g_check_open()
    248         file_ = parentnode._v_file

/usr/local/lib/python3.4/dist-packages/tables/node.py in _g_check_group(self, node)
    853         if not isinstance(node, class_name_dict['Group']):
    854             raise TypeError("new parent node ``%s`` is not a group"
--> 855                             % node._v_pathname)
    856 
    857     _g_checkGroup = previous_api(_g_check_group)

TypeError: new parent node ``/test_table`` is not a group
```

is there a way to append dataframe into table who was created using pytables? 
