title: How to specify metadata for dask.dataframe
body: <p>The docs provide good examples, <a href="http://dask.pydata.org/en/latest/dataframe-design.html#metadata" rel="noreferrer">how metadata can be provided</a>. However I still feel unsure, when it comes to picking the right dtypes for my dataframe.</p>
  
  <ul>
  <li>Could I do something like <code>meta={'x': int 'y': float,
  'z': float}</code> instead of <code>meta={'x': 'i8', 'y': 'f8', 'z': 'f8'}</code>?</li>
  <li>Could somebody hint me to a list of possible values like 'i8'?  What
  dtypes exist? </li>
  <li>How    can I specify a column, that contains arbitrary objects? How can I    specify a column, that contains only instances of one class?</li>
  </ul>