treebank: with TigerXML, require exactly one parent for each non-root node

- require each non-root node to have at least one parent <edge>
  to avoid disconnected trees (closes #21)
- disallow multiple <edge> with the same idref (but <secedge> may occur
  multiple times).

The latter constraint is violated by the Tiger XML corpus, at least in
v2.1 and v2.2. This appears to be an annotation/conversion error.
The following is a patch that removes the erroneous edges to follow the
v2.1 export version of the corpus:

https://gist.github.com/andreasvc/7284788