Enums support

[dnagir]

Ability to write something like:

class Filter
  include ActiveAttr::Model

  attribute :status, default: :all, enum: [:all, :available, :unavailable]
end

This would give us:

• Filter#status_name that is i8n translated based on the value of Filter#status.
• Filtet#status_hashes - the hash that can be used on the views.
• Assigning any valid outside of the given range would throw by default.
• Ability to use it with ActiveModel as well as ActiveRecord (and hooks for other ORMs if possible).

There's simple_enum and similar, but those seem to be tight to AR.

The implementation is pretty straightforward (for AR-only). This is what I'm currently using which may be a good start.

[johnnyshields]

This should be revisited with Enums now being added to Rails 4.1