Enable +/- ops to work between DatetimeIndex and DateOffset objects
```
In [1]: index = DatetimeIndex(['1/1/2000', '1/2/2000'])

In [2]: index
Out[2]: DatetimeIndex([2000-01-01 00:00:00, 2000-01-02 00:00:00], dtype=datetime64[us])

In [3]: index + datetools.Hour(2)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/code/pandas/<ipython-input-3-6142ead163c9> in <module>()
----> 1 index + datetools.Hour(2)

/home/wesm/code/pandas/pandas/core/index.pyc in wrapper(self, other)
   1142         else:
   1143             func = getattr(super(DatetimeIndex, self), opname)
-> 1144             return func(other)
   1145     return wrapper
   1146 

/home/wesm/code/pandas/pandas/core/index.pyc in __add__(self, other)
    412             return self.union(other)
    413         else:
--> 414             return Index(self.view(np.ndarray) + other)
    415 
    416     __eq__ = _indexOp('__eq__')

TypeError: ufunc 'add' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule 'safe'
```
