Equality comparison raises exception
Test case:

``` python
def test_datetimeindex__eq__():
    """Equality comparisons should never raise an exception"""
    pd.DatetimeIndex(['01-Jan-2015']) == ()
```

``` python
In [115]: test_datetimeindex__eq__()
Traceback (most recent call last):

  File "<ipython-input-115-9b7967ca9de2>", line 1, in <module>
    test_datetimeindex__eq__()

  File "<ipython-input-114-7ae5783187b8>", line 3, in test_datetimeindex__eq__
    pd.DatetimeIndex(['01-Jan-2015']) == ()

  File "C:\dev\bin\Anaconda\lib\site-packages\pandas\tseries\index.py", line 90, in wrapper
    other = _ensure_datetime64(other)

  File "C:\dev\bin\Anaconda\lib\site-packages\pandas\tseries\index.py", line 112, in _ensure_datetime64
    raise TypeError('%s type object %s' % (type(other), str(other)))

TypeError: <type 'tuple'> type object ()
```
