BUG: Comparisons with non-comparable objects in DataFrame return True
I can't tell if this is expected or not (taken from part of `test_where_datetime` in tests/test_frame). The change in #3311 (which the test case came from) didn't specify what should happen with non-datetime comparisons. Right now they return True, which doesn't really make sense. They should probably return False instead.

``` python
In [16]: df = DataFrame(dict(A = date_range('20130102',periods=5),
                            B = date_range('20130104',periods=5),
                            C = np.random.randn(5)))

In [17]: stamp = datetime(2013,1,3)

In [18]: df
Out[18]:
                    A                   B         C
0 2013-01-02 00:00:00 2013-01-04 00:00:00 -0.321163
1 2013-01-03 00:00:00 2013-01-05 00:00:00  1.219840
2 2013-01-04 00:00:00 2013-01-06 00:00:00 -1.048629
3 2013-01-05 00:00:00 2013-01-07 00:00:00 -0.861459
4 2013-01-06 00:00:00 2013-01-08 00:00:00  1.480088

In [19]: df > stamp
Out[19]:
       A     B     C
0  False  True  True
1  False  True  True
2   True  True  True
3   True  True  True
4   True  True  True

In [20]: df < stamp
Out[20]:
       A      B     C
0   True  False  True
1  False  False  True
2  False  False  True
3  False  False  True
4  False  False  True

In [21]: df == stamp
Out[21]:
       A      B      C
0  False  False  False
1   True  False  False
2  False  False  False
3  False  False  False
4  False  False  False

In [22]: df != stamp
Out[22]:
       A     B     C
0   True  True  True
1  False  True  True
2   True  True  True
3   True  True  True
4   True  True  True

In [23: pd.__version__
Out[23]: '0.12.0-154-g7fd6b20'
```

Column C probably should be False for every case.
