[#5] Add #nested_form method

From now user can use #nested_method on from classes. This works exactly
like old #nested_forms but there is no need to declarate two times
attribues.

I also removed copy error feature. Now we are using #validates_associated
method from ActiveRecord (with little changes).

Example of usage:

class UserForm < FormObjects::Base
   nested_form :addresses, Array[AddressForm]
   nested_form :personal_info, PersonalInfoForm
end

Fixes #5
Fixes #6
Merge pull request #7 from LTe/nested-form

[#5] Add #nested_form method