reindex fails on non-unique index
Since reindex and ix should behave the same way I guess since

"Some might conclude that ix and reindex are 100% equivalent based on this. This is indeed true except in the case of integer indexing."

The docstring also doesn't give any hint that one should use a unique index.

```
In [35]:

import pandas as pd
In [36]:

df = pd.DataFrame([('a',2), ('a',3), ('b',4)], columns = ['A', 'B'])
df2 = pd.DataFrame([('a',2), ('a',3), ('d',4)], columns = ['A', 'B'])
df.set_index('A', inplace=True)
df2.set_index('A', inplace=True)
Out[36]:
B
A   
a    2
a    3
d    4
In [37]:

df.ix[df.index.intersection(df2.index)]
Out[37]:
B
A   
a    2
a    3
In [38]:

df.reindex(df.index.intersection(df2.index))
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-38-ad4829c75d14> in <module>()
----> 1 df.reindex(df.index.intersection(df2.index))

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in reindex(self, index, columns, method, level, fill_value, limit, copy)
   2244         if index is not None:
   2245             frame = frame._reindex_index(index, method, copy, level,
-> 2246                                          fill_value, limit)
   2247 
   2248         return frame

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in _reindex_index(self, new_index, method, copy, level, fill_value, limit)
   2320                        limit=None):
   2321         new_index, indexer = self.index.reindex(new_index, method, level,
-> 2322                                                 limit=limit)
   2323         return self._reindex_with_indexers(new_index, indexer, None, None,
   2324                                            copy, fill_value)

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0-py2.7-linux-x86_64.egg/pandas/core/index.pyc in reindex(self, target, method, level, limit)
    825             else:
    826                 indexer = self.get_indexer(target, method=method,
--> 827                                            limit=limit)
    828         return target, indexer
    829 

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0-py2.7-linux-x86_64.egg/pandas/core/index.pyc in get_indexer(self, target, method, limit)
    746 
    747         if not self.is_unique:
--> 748             raise Exception('Reindexing only valid with uniquely valued Index '
    749                             'objects')
    750 

Exception: Reindexing only valid with uniquely valued Index objects

```
