title: Pandas pivot_table do not comply with values order
body: <p>I have an issue with pandas pivot_table.</p>
  
  <p>Sometimes, the order of the columns specified on "values" list does not match</p>
  
  <pre><code>In [11]: p = pivot_table(df, values=["x","y"], cols=["month"], 
                               rows="name", aggfunc=np.sum)
  </code></pre>
  
  <p>i get the wrong order (y,x) instead of (x,y)</p>
  
  <pre><code>Out[12]:
          y           x
  month   1   2   3   1   2   3
  name
  a       1 NaN   7   2 NaN   8
  b       3 NaN   9   4 NaN  10
  c     NaN   5 NaN NaN   6 NaN
  </code></pre>
  
  <p>Is there something i don't do well ?</p>