title: Should the pandas at accessor work with multiindexes?
body: <pre><code>def gen():
      yield '1','2',1,2
      yield '1','2',1,2
      yield '1','3',1,2
      yield '2','4',1,2
  
  df = pd.DataFrame(gen(), columns=["a", "b", "c", "d",]).set_index(["a", "b"])
  print df   # ('a','b') --&gt; ('c','d')
  </code></pre>
  
  <p>We have:</p>
  
  <pre><code>     c  d
  a b      
  1 2  1  2
    2  1  2
    3  1  2
  2 4  1  2
  </code></pre>
  
  <p>When accessing:</p>
  
  <pre><code>print df.loc[('1','3')]  # Success
  print df.at[('1','3')]   # KeyError: '3'
  </code></pre>
  
  <p>Also note, this also fails is df is a series:</p>
  
  <pre><code>print df['c'].at[('1','3')]   # TypeError: _get_value() got multiple values for keyword argument 'takeable'
  </code></pre>