Improve tables

This PR improves the support for tables. 

Now it will parse table borders 
```html
...
 < table border="x">....</ table>
...
```

And table headers
```html
...
<thead>
  <tr>
    <th> Header 1 </th>
    <th> Header 2 </th>
  </tr>
</thead> 
...
```

Generating tables that looks like 

![htmltoword_tables](https://cloud.githubusercontent.com/assets/169639/3429204/fad880e4-004f-11e4-97f2-92ccdbbfc97b.png)