title: Error with datetime column while serialising a DataFrame into an HDF5 store
body: <p>I'm trying to save a DataFrame into an HDF5 store using pandas builtin function to_hdf
  but this raises the following exception:</p>
  
  <blockquote>
    <p>File "C:\python\lib\site-packages\pandas\io\pytables.py", line 3433, in > >create_axes
       raise e
    TypeError: Cannot serialize the column [date] because
    its data contents are [datetime] object dtype</p>
  </blockquote>
  
  <p>the dataframe is built from a numpy array with correct types for each column</p>
  
  <p>I tried to convert_object() as I read in other frames but it still fails</p>
  
  <p>here is my test code, I am obviously missing something with the data conversion but cant figure out what</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  from datetime import datetime, timedelta
  
  columns = ['date', 'c1', 'c2']
  
  # building a sample test numpy array with datetime, float and integer
  dtype = np.dtype("datetime64, f8, i2")
  np_data = np.empty((0, len(columns)), dtype=dtype)
  for i in range(1, 3):
      line = [datetime(2015, 1, 1, 12, i), i/2, i*1000]
      np_data = np.append(np_data, np.array([line]), axis=0)
  print('##### the numpy array')
  print(np_data)
  
  # creating DataFrame from numpy array
  df = pd.DataFrame(np_data, columns=columns)
  # trying to force object conversion
  df.convert_objects()
  print('##### the DataFrame array')
  print(df)
  
  # the following fails!
  try:
      df.to_hdf('store.h5', 'data', append=True)
      print('worked')
  except Exception, e:
      print('##### the error')
      print(e)
  </code></pre>
  
  <p>The code above produces the following output</p>
  
  <pre><code>##### the numpy array
  [[datetime.datetime(2015, 1, 1, 12, 1) 0 1000]
   [datetime.datetime(2015, 1, 1, 12, 2) 1 2000]]
  ##### the DataFrame array
                    date c1    c2
  0  2015-01-01 12:01:00  0  1000
  1  2015-01-01 12:02:00  1  2000
  ##### the error
  Cannot serialize the column [date] because
  its data contents are [datetime] object dtype
  </code></pre>