DataFrame.pivot_table bug when rows is unspecified
```
In [6]: df
Out[6]: 
  cols rows  values
0    x    a       1
1    y    b       2
2    z    c       3

In [7]: df.pivot_table(rows='rows', aggfunc={'values': ['mean']})
Out[7]: 
      values
        mean
rows        
a          1
b          2
c          3

In [8]: df.pivot_table(cols='cols', aggfunc={'values': ['mean']})
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-f8b9a78d8e65> in <module>()
----> 1 df.pivot_table(cols='cols', aggfunc={'values': ['mean']})

/home/chang/code/pandas/pandas/tools/pivot.py in pivot_table(data, values, rows, cols, aggfunc, fill_value, margins)
    103                   for i in range(len(rows), len(keys))]
    104 
--> 105     table = agged.unstack(to_unstack)
    106 
    107     if isinstance(table, DataFrame):

/home/chang/code/pandas/pandas/core/frame.py in unstack(self, level)
   3947         """
   3948         from pandas.core.reshape import unstack
-> 3949         return unstack(self, level)
   3950 
   3951     #----------------------------------------------------------------------

/home/chang/code/pandas/pandas/core/reshape.pyc in unstack(obj, level)
    363 def unstack(obj, level):
    364     if isinstance(level, (tuple, list)):
--> 365         return _unstack_multiple(obj, level)
    366 
    367     if isinstance(obj, DataFrame):

/home/chang/code/pandas/pandas/core/reshape.pyc in _unstack_multiple(data, clocs)
    228     rlocs = [i for i in range(index.nlevels) if i not in clocs]
    229 
--> 230     clevels = [index.levels[i] for i in clocs]
    231     clabels = [index.labels[i] for i in clocs]
    232     cnames = [index.names[i] for i in clocs]

AttributeError: 'Index' object has no attribute 'levels'

```
