BUG: `concat` throwing TypeError when `ignore_index` is True
``` python
In [24]: dates = pd.date_range('01-Jan-2013', '01-Jan-2014', freq='MS')[0:-1]
    ...: s1 = pd.Series(randn(len(dates)), index=dates, name='value')    
    ...: s2 = pd.Series(randn(len(dates)), index=dates, name='value')


In [31]: df = pd.concat([s1, s2], axis=1, ignore_index=True)
 Traceback (most recent call last):

   File "<ipython-input-31-66b3acfe3258>", line 1, in <module>
     df = pd.concat([s1, s2], axis=1, ignore_index=True)

   File "c:\dev\code\pandas\pandas\tools\merge.py", line 895, in concat
     return op.get_result()

   File "c:\dev\code\pandas\pandas\tools\merge.py", line 972, in get_result
     data = dict(zip(self.new_axes[1], self.objs))

 TypeError: zip argument #1 must support iteration
```
