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?
This assertion should be split off into a separate context for the positive failure message.

Additionally, please use the `fail_with` matcher to test this, like so:

```
expect do
  expect(post).to define_enum_for(:condition)
end.to fail_with("Expected Post to have an enum :condition")
```