[new annotation proposal] Statements aka attribution

[proycon]

A span annotation type is needed to encode who says what about what/whom. This proposal is inspired on NAF's attribution layer:

<s>
 <w xml:id="w1"><t>They</t></w>
 <w xml:id="w2"><t>said</t></w>
 <w xml:id="w3"><t>the</t></w>
 <w xml:id="w4"><t>hotel</t></w>
 <w xml:id="w5"><t>was</t></w>
 <w xml:id="w6"><t>a</t></w>
 <w xml:id="w7"><t>nightmare</t></w>
 <w xml:id="w8"><t>.</t></w>
 <statements>
  <statement class="said">
   <source>
    <wref id="w1" />
   </source>
   <hd>
    <wref id="w3" />
    <wref id="w4" />
    <wref id="w5" />
    <wref id="w6" />
    <wref id="w7" />
   </hd>
   <relation>
     <wref id="w2" />
   </relation>
  </statement>
 </statements>
</s>

This introduces/uses the following span roles:

• hd - The statement itself (required)
• source - The source/holder of the statement (optional), used also in issue [new annotation proposal] Sentiment Analysis #16
• relation - The relationship between source and the head (optional)

Whether the statement's class expresses the relationship like in the example, or has more direct bearing on the statement itself, is of course up to the set used.