title: Select only available rows of a pandas dataframe
body: <p>Let say I have the following <code>pandas df</code></p>
  <pre><code>import pandas as pd
  d = [0.0, 1.0, 2.0]
  e = pd.Series(d, index = ['a', 'b', 'c'])
  df = pd.DataFrame({'A': 1., 'B': e, 'C': pd.Timestamp('20130102')})
  </code></pre>
  <p>Now I have another array</p>
  <pre><code>select = ['c', 'a', 'x']
  </code></pre>
  <p>Clearly, the element <code>'x'</code> is not available in my original <code>df</code>. How can I select rows of <code>df</code> based on <code>select</code> but choose only available rows without any error? i.e. in this case, I want to select only rows corresponding to <code>'c'</code> and <code>'a'</code> maintaining this order.</p>
  <p>Any pointer will be very helpful.</p>