title: Filter on Pandas DataFrame with datetime columns raises error
body: <p>I'm setting up a DataFrame with two datetime columns like so:</p>
  
  <pre><code>range1 = Series(date_range('1/1/2011', periods=50, freq='D'))
  range2 = Series(date_range('2/5/2011', periods=50, freq='D'))
  df1    = DataFrame({'a': rng1, 'b': rng2}, dtype='datetime64[D]')
  </code></pre>
  
  <p>Oddly, asking the dtypes of df1 gives me:</p>
  
  <pre><code>In [71]: df1.dtypes
  Out[71]:
  a    datetime64[ns]
  b    datetime64[ns]
  </code></pre>
  
  <p>What is worse, when I try to filter the dataframe like so:</p>
  
  <pre><code>In [62]:
  
  d = datetime(2011,1,14)
  df1[df1 &gt; d]
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-62-50b4b9735157&gt; in &lt;module&gt;()
        1 d = datetime(2011,1,14)
  ----&gt; 2 df1[df1 &gt; d]
  
  /usr/local/lib/python2.7/site-packages/pandas/core/frame.pyc in f(self, other)
      313             return self._combine_series_infer(other, func)
      314         else:
  --&gt; 315             return self._combine_const(other, func)
      316 
      317     f.__name__ = name
  
  /usr/local/lib/python2.7/site-packages/pandas/core/frame.pyc in _combine_const(self, other, func)
     3677             return self
     3678 
  -&gt; 3679         result_values = func(self.values, other)
     3680 
     3681         if not isinstance(result_values, np.ndarray):
  
  TypeError: can't compare datetime.datetime to long
  </code></pre>
  
  <p>Could anyone tell me why this is happening? I am using numpy 1.7 and pandas 0.10.1.</p>