BUG: string output on PeriodIndex fails
used to work in 0.9.1.

this is on current master

```
In [1]: import numpy as np

In [2]: import pandas as pd

In [3]: index = pd.PeriodIndex(['2011-1', '2011-2', '2011-3'], freq='M')

In [4]: frame = pd.DataFrame(np.random.randn(3,4),index=index)

In [5]: frame.to_string()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-b6219037419a> in <module>()
----> 1 frame.to_string()

/mnt/home/jreback/pandas/pandas/core/frame.pyc in to_string(self, buf, columns, col_space, colSpace, header, index, na_rep, formatters, float_format, sparsify, nanRep, index_names, justify, force_unicode, line_width)
   1501                                            header=header, index=index,
   1502                                            line_width=line_width)
-> 1503         formatter.to_string()
   1504 
   1505         if buf is None:

/mnt/home/jreback/pandas/pandas/core/format.pyc in to_string(self, force_unicode)
    295             text = info_line
    296         else:
--> 297             strcols = self._to_str_columns()
    298             if self.line_width is None:
    299                 text = adjoin(1, *strcols)

/mnt/home/jreback/pandas/pandas/core/format.pyc in _to_str_columns(self)
    240 
    241         # may include levels names also
--> 242         str_index = self._get_formatted_index()
    243         str_columns = self._get_formatted_column_labels()
    244 

/mnt/home/jreback/pandas/pandas/core/format.pyc in _get_formatted_index(self)
    444                                      formatter=fmt)
    445         else:
--> 446             fmt_index = [index.format(name=show_index_names, formatter=fmt)]
    447 
    448         adjoined = adjoin(1, *fmt_index).split('\n')

TypeError: format() got an unexpected keyword argument 'formatter'

```
