Refactor/user

User can now be instantiated with a uid

    >>> u = User.register("mek", "password")
    <User {'username': 'mek', 'enabled': True, 'email': ''}>
    >>> u.username
    'mek'
    >>> u2 = User('mek')
    <User {'username': 'mek', 'enabled': True, 'email': ''}>

Also added User.getall() classmethod

    >>> User.getall()