[#2] Add json serializer

You can serialize attributes by #as_json or #to_json. In this way you
can in simple way pass attributes to services objects.

Fixes #2
Merge pull request #10 from LTe/serializer

[#2] Add json serializer
[#4] Add alias for #attribute method

More readable in context of forms is #field than #attribute. Because in
forms more natural is field than attribute. Attribute is more natural to
objects (aggregates, models, orms).

Fixes #4