Allow Timeseries-like shortcuts in dataframes __getitem__
``` python
In [17]: s=pd.date_range("2001-1-1", periods=12)
In [20]: ts['2001']
Out[20]: 
2001-01-01     0
2001-01-02     1
2001-01-03     2
2001-01-04     3
2001-01-05     4
2001-01-06     5
2001-01-07     6
2001-01-08     7
2001-01-09     8
2001-01-10     9
2001-01-11    10
2001-01-12    11
Freq: D, dtype: int64

In [24]: df=pd.DataFrame(range(len(s)),index=s)
In [25]: df['2001']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
KeyError: u'no item named 2001'

In [27]: df.index is ts.index
Out[27]: True
```

related #2091
