Update db.py

allow to define db_host in config file (when MySQL server is not local)
Update wnf.py

use db_host in config file for mysql command
Update backups.py

use conn.host parameter and correction with get_backup method
Update requirements.txt

add slugify (used to handle unicode file names)
Merge branch 'master' of github.com:miguelfontanes/wnframework into miguelfontanes-master
[minor] [fix] backup