ActiveRecord::Enum matcher

Hi guys,

I started a matcher for the  [ActiveRecord::Enum](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Enum.html). I know we can`t merge this now since no released rails 4.1.0.

I only want to show you guys the idea =) and ask one thing: ActiveRecord::Enum is only on rails 4.1.0, how we can skip this specs in older rails versions?
Let's rename this to `expected_enum_values`. This way:

* to access the *original* `expected_enum_values` you can use `@options[:expected_enum_values]`
* to access the expected enum values as an indifferent hash, use `expected_enum_values`