pivot_table with margins=True fails
Coming from http://stackoverflow.com/q/19300730/624829

<pre>
import pandas as pd
values = pd.Series(i for i in range(5))
rows = pd.Series(['a', 'b', 'a', 'a', 'b'])
columns = pd.date_range('20130101',periods=5)

df = pd.DataFrame({'values': values, 'rows': rows, 'columns': columns})

pivot = pd.pivot_table(
    data=df,
    rows='rows',
    cols='columns',
    values='values',
    margins=True
)
</pre>


This call fails down to that traceback:

<pre>
Traceback (most recent call last):
  File "./test.py", line 17, in <module>
    margins=True
  File "/usr/local/lib/python2.6/dist-packages/pandas/tools/pivot.py", line 135, in pivot_table
    cols=cols, aggfunc=aggfunc)
  File "/usr/local/lib/python2.6/dist-packages/pandas/tools/pivot.py", line 174, in _add_margins
    piece[all_key] = margin[key]
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/frame.py", line 2119, in __setitem__
    self._set_item(key, value)
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/frame.py", line 2166, in _set_item
    NDFrame._set_item(self, key, value)
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/generic.py", line 679, in _set_item
    self._data.set(key, value)
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/internals.py", line 1781, in set
    self.insert(len(self.items), item, value)
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/internals.py", line 1801, in insert
    new_items = self.items.delete(loc)
  File "/usr/local/lib/python2.6/dist-packages/pandas/core/index.py", line 2610, in delete
    new_labels = [np.delete(lab, loc) for lab in self.labels]
  File "/usr/lib/pymodules/python2.6/numpy/lib/function_base.py", line 3339, in delete
    "invalid entry")
ValueError: invalid entry
</pre>


Resetting the columns attribute on the df before workarounds the issue:

<pre>
df.columns = ['rows', 'columns', 'values']
</pre>
