tables and attr_list compatibility

Treats `<td>` as an inline element even though it's a block element. As far as I know, there's no way to have multi-line cells with the tables extension, and simply using an html `<td>` gets wrapped in an additional `<td>`, so I think there's a case for an exception. [This deficiency has practical consequences.](http://stackoverflow.com/questions/18901540/how-do-i-add-attributes-to-tables-in-python-markdown/19615075#19615075)

Example:
```
import markdown


example = """\
First Header                                           | Second Header
------------------------------------------------------ | -------------
Content Cell{: class="foo bar" title="Some title!" }   | Content Cell
Content Cell                                           | Content Cell{: class="foo bar" title="Some title!" }
"""

output = markdown.markdown(example, extensions=['extra'])
print """-----------------------------------------------
EXAMPLE OUTPUT:
    %s
-------------------------------------------------""" % output
```

Output Before:
```
-----------------------------------------------
EXAMPLE OUTPUT:
    <table>
<thead>
<tr>
<th>First Header</th>
<th>Second Header</th>
</tr>
</thead>
<tbody>
<tr>
<td>Content Cell{: class="foo bar" title="Some title!" }</td>
<td>Content Cell</td>
</tr>
<tr>
<td>Content Cell</td>
<td>Content Cell{: class="foo bar" title="Some title!" }</td>
</tr>
</tbody>
</table>
-------------------------------------------------
```

Output After:
```
-----------------------------------------------
EXAMPLE OUTPUT:
    <table>
<thead>
<tr>
<th>First Header</th>
<th>Second Header</th>
</tr>
</thead>
<tbody>
<tr>
<td class="foo bar" title="Some title!">Content Cell/n</td>
<td>Content Cell</td>
</tr>
<tr>
<td>Content Cell</td>
<td class="foo bar" title="Some title!">Content Cell</td>
</tr>
</tbody>
</table>
-------------------------------------------------
```