BUG: HDFStore fails to save panel with mixed type minor axis when format='table'
I'm trying to save a panel that has a mixed type minor axis using HDFStore. I can save the panel when using the fixed format (although I get a PerformanceWarning), but it fails for the table format. Example code:

``` python
import pandas as pd
import numpy as np

d1 = pd.DataFrame({1:pd.Series(np.random.randn(10)), 2:pd.Series(np.random.randn(10))})
d2 = pd.DataFrame({1:pd.Series(np.random.randn(10)), 2:pd.Series(np.random.randn(10))})
good_panel = pd.Panel({'a':d1, 'b':d2})
print good_panel

with pd.get_store('/tmp/goodstore.h5', mode='w') as store:
    store.put('/test', good_panel, 'table', append=False)

b1 = pd.DataFrame({1:pd.Series(np.random.randn(10)), 'a':pd.Series(np.random.randn(10))})
b2 = pd.DataFrame({1:pd.Series(np.random.randn(10)), 'a':pd.Series(np.random.randn(10))})
bad_panel = pd.Panel({'a':b1, 'b':b2})
print bad_panel

with pd.get_store('/tmp/badstore.h5', mode='w') as store:
    store.put('/test', bad_panel, 'fixed', append=False)
print 'saved as fixed'

with pd.get_store('/tmp/badstore.h5', mode='w') as store:
    store.put('/test', bad_panel, 'table', append=False)
```

The error I get is:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-3528d8319daf> in <module>()
      9 
     10 with pd.get_store('/tmp/badstore.h5', mode='w') as store:
---> 11     store.put('/test', bad_panel, 'table', append=False)

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/pandas/io/pytables.pyc in put(self, key, value, format, append, **kwargs)
    829             format = get_option("io.hdf.default_format") or 'fixed'
    830         kwargs = self._validate_format(format, kwargs)
--> 831         self._write_to_group(key, value, append=append, **kwargs)
    832 
    833     def remove(self, key, where=None, start=None, stop=None):

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/pandas/io/pytables.pyc in _write_to_group(self, key, value, format, index, append, complib, encoding, **kwargs)
   1278 
   1279         # write the object
-> 1280         s.write(obj=value, append=append, complib=complib, **kwargs)
   1281 
   1282         if s.is_table and index:

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/pandas/io/pytables.pyc in write(self, obj, axes, append, complib, complevel, fletcher32, min_itemsize, chunksize, expectedrows, dropna, **kwargs)
   3657 
   3658             # create the table
-> 3659             table = self._handle.create_table(self.group, **options)
   3660 
   3661         else:

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/tables/file.pyc in create_table(self, where, name, description, title, filters, expectedrows, chunkshape, byteorder, createparents, obj)
   1065                       description=description, title=title,
   1066                       filters=filters, expectedrows=expectedrows,
-> 1067                       chunkshape=chunkshape, byteorder=byteorder)
   1068 
   1069         if obj is not None:

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/tables/table.pyc in __init__(self, parentnode, name, description, title, filters, expectedrows, chunkshape, byteorder, _log)
    805         if new and isinstance(description, dict):
    806             # Dictionary case
--> 807             self.description = Description(description)
    808         elif new and (type(description) == type(IsDescription)
    809                       and issubclass(description, IsDescription)):

/home/tony/.virtualenvs/dev2/lib/python2.7/site-packages/tables/description.pyc in __init__(self, classdict, nestedlvl, validate)
    493                                 'got: "%s". Please make use of the Col(), or '
    494                                 'descendant, constructor to properly '
--> 495                                 'initialize columns.' % object)
    496             object._v_pos = pos  # Set the position of this object
    497             object._v_parent = self  # The parent description

TypeError: Passing an incorrect value to a table column. Expected a Col (or subclass) instance and got: "ObjectAtom()". Please make use of the Col(), or descendant, constructor to properly initialize columns.
```

My system info:

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-431.17.1.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.15.0
nose: 1.3.4
Cython: 0.20.1
numpy: 1.9.0
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 2.3.0
sphinx: 1.2.2
patsy: 0.2.1
dateutil: 2.2
pytz: 2014.7
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.0
openpyxl: 1.8.6
xlrd: None
xlwt: None
xlsxwriter: None
lxml: 3.3.5
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.4
pymysql: None
psycopg2: None
```
