fix mdx_urlize#31

``` Traceback (most recent call last): File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response response = callback(request, *callback_args, **callback_kwargs) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 20, in _wrapped_view return view_func(request, *args, **kwargs) File "/home/mherrero/webapps/ltmo/src/ltmo/ltmo/views.py", line 45, in edit leak = form.save() File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/django/forms/models.py", line 364, in save fail_message, commit, construct=False) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/django/forms/models.py", line 86, in save_instance instance.save() File "/home/mherrero/webapps/ltmo/src/ltmo/ltmo/models.py", line 38, in save ['video', 'codehilite', 'urlize'] File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/markdown/__init__.py", line 412, in markdown return md.convert(text) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/markdown/__init__.py", line 296, in convert newRoot = treeprocessor.run(root) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/markdown/treeprocessors.py", line 284, in run text), child) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/markdown/treeprocessors.py", line 105, in __handleInline data, patternIndex, startIndex) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/markdown/treeprocessors.py", line 232, in __applyPattern node = pattern.handleMatch(match) File "/home/mherrero/webapps/ltmo/lib/python2.7/site-packages/mdx_urlize.py", line 88, in handleMatch el = markdown.etree.Element("img") AttributeError: 'module' object has no attribute 'etree' ```