title: How to sort a Series or DataFrame by a given index order?
body: <p>Suppose I have a Series like this:</p>
  
  <pre><code>In [19]: sr
  Out[19]: 
  a    1
  b    2
  c    3
  d    4
  dtype: int64
  
  In [20]: sr.index
  Out[20]: Index([u'a', u'b', u'c', u'd'], dtype='object')
  </code></pre>
  
  <p>Instead of sorting lexicographically, I would like to sort this series based on a custom order, say, <code>cdab</code>. How can I do that?</p>
  
  <p>What if it is a DataFrame; how can I sort it by a given index list?</p>