Replace RawQuery with compiler.results_iter()

This originated with behavior seen in Avocado where data was being shifted.
It was discovered to be the Oracle ROWNUM identifier that was prepended to
each row. See chop-dbhi/avocado#98

Fix #8

modeltree/query.py

@@ -1,5 +1,4 @@
 from django.db.models import query
-from django.db.models.sql import RawQuery
 from modeltree.tree import trees
 from modeltree.utils import M
 
@@ -33,5 +32,5 @@ def select(self, *fields, **kwargs):
         return self.tree.add_select(queryset=queryset, *fields, **kwargs)
 
     def raw(self):
-        sql, params = self.query.get_compiler(self.db).as_sql()
-        return RawQuery(sql, self.db, params)
+        compiler = self.query.get_compiler(self.db)
+        return compiler.results_iter()