error during line-wise iteration over DataFrame
In the code below I create a `DataFame` and iterate over its lines using `ix`

```
In [37]: a = pandas.DataFrame(rand(4, 2), columns = list('ab'))

In [38]: print a
          a         b
0  0.372630  0.680347
1  0.557162  0.815903
2  0.058282  0.371265
3  0.772931  0.925008

In [39]: for l in a.ix:
   ....:     print l
   ....:
a    0.372630
b    0.680347
a    0.557162
b    0.815903
Name: 1
a    0.058282
b    0.371265
Name: 2
a    0.772931
b    0.925008
Name: 3
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Q:\Groups\Bioinformatics\IgG assay optimization 2011\script development\scriptDevelopment\src\<ipython-input-39-50ae06db9052> in <module>()
----> 1 for l in a.ix:
      2     print l
      3

c:\python27\lib\site-packages\pandas\core\indexing.pyc in __getitem__(self, key)
     30             return self._getitem_tuple(key)
     31         else:
---> 32             return self._getitem_axis(key, axis=0)
     33
     34     def _get_label(self, label, axis=0):

c:\python27\lib\site-packages\pandas\core\indexing.pyc in _getitem_axis(self, key, axis)
    182                     idx = labels[key]
    183
--> 184             return self._get_label(idx, axis=0)
    185         else:
    186             labels = self.obj._get_axis(axis)

c:\python27\lib\site-packages\pandas\core\indexing.pyc in _get_label(self, label, axis)
     36             return self.obj.xs(label, axis=axis, copy=False)
     37         except Exception:
---> 38             return self.obj.xs(label, axis=axis, copy=True)
     39
     40     def _slice(self, obj, axis=0):

c:\python27\lib\site-packages\pandas\core\frame.pyc in xs(self, key, axis, level, copy)
   1592             loc, new_index = self.index.get_loc_level(key)
   1593         else:
-> 1594             loc = self.index.get_loc(key)
   1595
   1596         if np.isscalar(loc):

c:\python27\lib\site-packages\pandas\core\index.pyc in get_loc(self, key)
    493         loc : int
    494         """
--> 495         return self._engine.get_loc(key)
    496
    497     def get_value(self, series, key):

c:\python27\lib\site-packages\pandas\_engines.pyd in pandas._engines.DictIndexEngine.get_loc (pandas\src\engines.c:2498)()

c:\python27\lib\site-packages\pandas\_engines.pyd in pandas._engines.DictIndexEngine.get_loc (pandas\src\engines.c:2460)()

KeyError: 4
```
