title: Pandas HDF5 append time series fails
body: <p>Going through the documentation of pandas HDF5 usability (<a href="http://pandas.pydata.org/pandas-docs/stable/io.html#io-hdf5" rel="nofollow">http://pandas.pydata.org/pandas-docs/stable/io.html#io-hdf5</a>) the given example raises an error:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  store = pd.HDFStore('store.h5')
  
  np.random.seed(1234)
  
  index = pd.date_range('1/1/2000', periods=8)
  
  df = pd.DataFrame(np.random.randn(8, 3), index=index)
  
  store['df'] = df
  
  df1 = df[0:4]
  df2 = df[4:]
  store.append('df', df1)
  store.append('df', df2)
  
  Traceback (most recent call last):
    File "C:\Anaconda3\lib\site-packages\IPython\core\interactiveshell.py", line 2885, in run_code
      exec(code_obj, self.user_global_ns, self.user_ns)
    File "&lt;ipython-input-225-ef7f2e059c6a&gt;", line 1, in &lt;module&gt;
      store.append('df', df1)
    File "C:\Anaconda3\lib\site-packages\pandas\io\pytables.py", line 919, in append
      **kwargs)
    File "C:\Anaconda3\lib\site-packages\pandas\io\pytables.py", line 1252, in _write_to_group
      raise ValueError('Can only append to Tables')
  ValueError: Can only append to Tables
  </code></pre>
  
  <p>Has something changed here? Or am I doing something wrong?</p>