title: Pandas _metadata of DataFrame persistence error
body: <p>I have finally figured out how to use _metadata from a DataFrame, everything works except I am unable to persist it such as to hdf5 or json.  I know it works because I copy the frame and _metadata attributes copy over "non _metadata" attributes don't.</p>
  
  <p>example</p>
  
  <pre><code>df = pandas.DataFrame #make up a frame to your liking
  pandas.DataFrame._metadata = ["testmeta"]
  df.testmeta = "testmetaval"
  df.badmeta = "badmetaval"
  newframe = df.copy()
  newframe.testmeta --&gt;outputs "testmetaval"
  newframe.badmeta ---&gt; raises attribute error
  
  #json test
  df.to_json(Path)
  revivedjsonframe = pandas.io.json.read_json(Path)
  revivedjsonframe.testmeta ----&gt;raises Attribute Error
  
  #hdf5 test
  revivedhdf5frame.testmeta ---&gt; returns None
  </code></pre>
  
  <p>this person <a href="https://stackoverflow.com/a/25715719/4473236">https://stackoverflow.com/a/25715719/4473236</a> says it worked for him but I'm new to this site (and pandas) and can't post to that thread or ask him directly.</p>