Fix RDFa hierarchy for tree.py

[jamesaoverton]

The RDFa/HTML generated by the current code doesn't parse into the right subclass hierarchy. Messing around with the RDFa Play site http://rdfa.info/play/ I think the structure should be more like:

<li>
 <a about="owl:Thing" rev="rdfs:subClassOf" resource="BFO:0000001" href="?id=BFO:0000001">entity</a>
  <ul>
    <li>
      <a about="BFO:0000001" rev="rdfs:subClassOf" resource="BFO:0000002" href="?id=BFO:0000002">continuant</a>
      <ul>
        <li>
          <a about="BFO:0000002" rev="rdfs:subClassOf" resource="BFO:0000004" href="?id=BFO:0000004">independent continuant</a>
          
....

I'd like the tests to check this, too.