Two small modifications

I've done two changes which I think are generally useful:

1. Added the method `Value.get_actual_value`: it returns the actual value stored in the database, which is useful when populating a form.

2. Modified the method `Entity.get_value_by_attribute` by adding default value, which is `None` by default. I don't see a point in throwing an exception when an attribute is not set; a more often use case is that one will iterate through the attributes to get their values, and default comes handy in those cases.