Port to Python 2.4~2.6 and 3.0~3.3

Known bug: different indentation in Python 2.6:

``` python
{'root': {
    'test': 'test'
}}
```

generates:

``` xml
<?xml version="1.0" ?>
<root>
  <test>
    test
  </test>
</root>
```

instead of:

``` xml
<?xml version="1.0" ?>
<root>
  <test>test</test>
</root>
```
