Provide decorators as an alternative to magic methods

[MatthieuDartiailh]

Following the addition of support for typing in the last release, I think that adding the following decorators would help since the decorators will provide type hints when it comes to the expected signature of the method. Additionally it will allow more easily to share the same handler across several members by accepting several names. Using a decorator also make the intent very clear and will allow the metaclass to raise rather than simply warn (see #139 ) if the member does not exist.

• default
• post_setattr
• post_getattr
• validate
• post_validate