solnic
Added method attribute_names
Added little but useful method attribute_names for Virtus model. There are a lot of cases when we need to get attribute names in a simple way.
Sorry, I have very little time for Virtus these days.
Regarding this addition - I would accept it if it was a method defined on attribute_set itself. ie AttributeSet#names so that you could do YourClass.attribute_set.names.
We want to add as little as possible to the interface of the class or module that includes a virtus extension.
My first thought was that attribute_names would be useful, and doesn't pollute the model's namespace any worse than attributes. But since attributes is deprecated, I think @solnic is right --- we should implement it as attribute_set.names instead.
I find ActiveRecord's attribute_names useful, but it doesn't seem to be required to use ActiveModel.
