fixed unicode issue with _escape() in yamdwe_users.py

After successfully moved content from MediaWiki to DokuWiki, I run into this issue, when I executed yamdwe_users.py. I got following error message in line 97:
> TypeError: decoding Unicode is not supported
After a little research I found that solution, which worked for me. I assume this error happened, because usernames from MediaWiki contains non ASCII characters.
As I'm not a Python programmer, please check this patch carefully. 

My environment:
Ubuntu 14.04
MediaWiki 1.22.5
DokuWiki 2014-09-29a "Hrun"
Python 2.7.6
MySQL 5.5.40
DB Table Collations utf8_general_ci
user_name and user_real_name are VARCHAR(255) utf8_bin

Armin