Series doesn't have an is_unique attribute
Hello,

Series doesn't have is a `is_unique` attribute

Index have a `is_unique` attribute but not (values of a) Series

``` python
>>> df.index.is_unique
True

>>> df['Column1'].is_unique
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-417-c450d4c4ceda> in <module>()
----> 1 df['Date'].is_unique

//anaconda/envs/py27/lib/python2.7/site-packages/pandas/core/generic.pyc in __getattr__(self, name)
   2358                 return self[name]
   2359             raise AttributeError("'%s' object has no attribute '%s'" %
-> 2360                                  (type(self).__name__, name))
   2361
   2362     def __setattr__(self, name, value):

AttributeError: 'Series' object has no attribute 'is_unique'
```

I'm doing this

``` python
def is_unique(ser):
    return len(np.unique(ser.values)) == len(ser.values)
```

but this kind of attribute could be useful when you want to set a column as index and ensure that there is no duplicate keys.

PS: I know there is

``` python
    df.set_index('Column1', verify_integrity=True)
```
