Fix image upload when IMAGE_PATH_OBSCURIFY setting is enabled

Fixes the following bug when using Python 3:

```
Request Method:	POST
Request URL:	http://127.0.0.1:8000/wiki/_edit/?f=form0
Django Version:	1.6.2
Exception Type:	TypeError
Exception Value:	
Unicode-objects must be encoded before hashing
Exception Location:	/Users/antonin/Documents/Projects/python/src/django-wiki/wiki/plugins/images/models.py in upload_path, line 24
Python Executable:	/Users/antonin/Documents/Projects/python/bin/python3
Python Version:	3.3.5
```

As you can see [here](http://stackoverflow.com/a/20060712/835351), the output is exactly the same length as md5 and is unique.