Implemented #14, implemented most of #16

- Added hasing for passwords (#14)
- Removed default user accounts. There is only one default account now
with name "admin" and PW "admin". This account should be used to load
user data from xml and then be disabled.
- Started implementing #16 user management for admin. Admin can change
names, change roles, generate new password, add new user
- User can have a null password hash now. These accounts work like
usual, but can't be used by their users for loggin. When a new user is
created, he has no password (null) until admin generates one.