python memory leak in dom.TreeBuilder

```
html5lib/treebuilders/dom.py

on line 122:

def documentClass(self):
    self.dom = Dom.getDOMImplementation().createDocument(None,None,None)
    return self



and in file html5lib/treebuilders/_base.py
on line 131:

self.document = self.documentClass()

circular reference!

```

Original issue reported on code.google.com by `devex.s...@gmail.com` on 9 Oct 2009 at 5:56
