[#3] Add #update method

You can use @object.update(params) insteaf of #attributes=
Merge pull request #9 from LTe/attributes-update

[#3] Add #update method
[#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