Refactored urlize_quoted_links based on Dango 1.5 urlize, which is more robust than the current one

Base ```urlize_quoted_links``` code in Django 1.5 ```urlize```, see issue #763

Also cleaned a bit unused variables like:

```python
link_target_attribute_re
html_gunk_re
hard_coded_bullets_re
trailing_empty_content_re
....
```