Semantic table HTML reader

[apapsch]

This pull request introduces the new reader PHPExcel_Reader_HTML_SemanticTable. Compared to the existing HTML reader, SemanticTable doesn't try to convert everything table-looking. Instead only table elements are recognized and each is treated as a worksheet with the children elements caption, thead and tbody parsed as worksheet title, header rows and content rows respectively.

For example, the following HTML portion is recognized by SemanticTable:

<table>
  <caption>Worksheet name</caption>
  <thead>
    <tr>
      <th>Heading in A1<th>
      <th>Heading in B1</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Datenspalte A2</td>
      <td>Datenspalte B2</td>
    </tr>
    <tr>
      <td>Datenspalte A3</td>
      <td>Datenspalte B3</td>
    </tr>
  </tbody>
</table>

To avoid repetitions in PHPExcel_Reader_HTML and PHPExcel_Reader_HTML_SemanticTable, an abstract base class is introduced which is used for common functionality. Care was taken to keep modifications in PHPExcel_Reader_HTML at a minimum, therefore it only uses defaultElementHandler and doesn't use TRAVERSE_CHILDS hint.