Implement request #301: Create users when using external auth login

After a successful login with imap, smb or ftp, the corresponding
user is created in the database.
This is needed to make e.g. sharing files with this users work.

A random password is used, so that the original password is kept in the
original location and is not duplicated here. Login is only possible
with the original password.

See https://github.com/owncloud/apps/issues/301 and
https://github.com/owncloud/apps/issues/302#issuecomment-14663829