BUG: Creating pivot with margins crashes
I'm trying to create simple pivot

``` python
import pandas as pd
import numpy
import datetime

s1 = pd.Series([datetime.datetime.now() - datetime.timedelta(days=i) for i in range(5)])
s2 = pd.Series(['a1', 'a1', 'a2', 'a2', 'a3'])
s3 = pd.Series([i for i in range(5)])
frame = pd.DataFrame({'s1': s1, 's2': s1, 's3': s3})
pivot = pd.pivot_table(frame, rows='s2', cols='s1', values='s3', margins=True)
```

It happens if I set "margins=True" and cols for pivot is datetime.

```
Traceback (most recent call last):
  File "./aa.py", line 12, in <module>
    p = pd.pivot_table(b, rows='s2', cols='s1', values='s3', 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/local/lib/python2.6/dist-packages/numpy/lib/function_base.py", line 3456, in delete
    "invalid entry")
ValueError: invalid entry
```
