datetime64[ns] index values output incorrect in DataFrame.to_html()
Please see the example below:

``` python
In [39]: df = pd.DataFrame({'a': np.arange(3)}, index=pd.date_range('2010-01-01', periods=3, freq='B'))

In [40]: print df
            a
2010-01-01  0
2010-01-04  1
2010-01-05  2

In [41]: print df.to_html()
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>a</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td><strong>1970-01-15 48:00:00</strong></td>
      <td> 0</td>
    </tr>
    <tr>
      <td><strong>1970-01-15 120:00:00</strong></td>
      <td> 1</td>
    </tr>
    <tr>
      <td><strong>1970-01-15 144:00:00</strong></td>
      <td> 2</td>
    </tr>
  </tbody>
</table>

In [42]: print pd.version.version
0.9.0.dev-a884949

In [43]: print np.version.version
1.6.1

In [44]: df.index.dtype
Out[44]: dtype('datetime64[ns]')
```
