Finish and clean up Python 3 support

- Handle Python 3 types similar to libxml2
- Copy new versions of libxml_xmlXPathDestructNsNode and
  libxml_xmlXPathObjectPtrConvert from libxml2
- Fix compiler warnings
- Fix whitespace
- Remove unneeded imports from __future__
- Remove test in extelem.py (StringIO can't be converted to FILE under
  Python 3)
- string.lower() works in both Python 2 and 3

Closes #25.

python/tests/extfunc.py

@@ -1,6 +1,4 @@
 #!/usr/bin/python -u
-from __future__ import print_function
-
 import sys
 import string
 import libxml2
@@ -24,7 +22,7 @@ def f(ctx, str):
     except:
         pass
 
-    return string.upper(str)
+    return str.upper()
 
 libxslt.registerExtModuleFunction("foo", "http://example.com/foo", f)