Django authentification backend improvements

- add support for more password hashers: md5, bcrypt, bcrypt_sha256
- some bugfixes
- django super/staff user as owncloud admin now works with owncloud 6
- the django database does nolonger need to be the same as the owncloud database. the owncloud admin needs to configure a separate database connection. database settings are stored in config.php.
- removed some typos
- code consistency
Singleton is an antipattern, use dependency injection for creating only one instance http://doc.owncloud.org/server/7.0/developer_manual/app/container.html