to_html: sparsify mutliindex keys in dataframe rows
The sparsify option of DataFrame.to_html currently works only for columns. It would be useful to add this option also for rows, similarly to what is already done for data frame representation in console.

Example:

``` python
df = pd.DataFrame(np.arange(3), index=[['X', 'X', 'Y'], ['a','b','a']])
df.to_html(fid)
```

currently produces the following output:

<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>0</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td><strong>X</strong></td>
      <td><strong>a</strong></td>
      <td> 0</td>
    </tr>
    <tr>
      <td><strong>X</strong></td>
      <td><strong>b</strong></td>
      <td> 1</td>
    </tr>
    <tr>
      <td><strong>Y</strong></td>
      <td><strong>a</strong></td>
      <td> 2</td>
    </tr>
  </tbody>
</table>


This should be changed to:

<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>0</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td rowspan="2" valign="top"><strong>X</strong></td>
      <td><strong>a</strong></td>
      <td> 0</td>
    </tr>
    <tr>
      <td><strong>b</strong></td>
      <td> 1</td>
    </tr>
    <tr>
      <td><strong>Y</strong></td>
      <td><strong>a</strong></td>
      <td> 2</td>
    </tr>
  </tbody>
</table>

