Implemented multiple login manager with a prefix to many keys

current_user is a property of LoginManager now.

Moved following global functions to a method of LoginManager without
parameter or return value changes:
    login_fresh
    login_user
    logout_user
    confirm_login
    login_required
    fresh_login_required
    _get_user
    _user_context_processor

Does not attach the login manager to Flask app anymore because there
could be more than one instances working.

Unittest passed ... with some little changes :)