Add the ability to nullize the enum column

In the implementation of Enum in Rails 4.1.0.beta1, you **can't** set an enum to `nil`.

I just monkey-patched the `ActiveRecord::Enum.enum`, here: https://gist.github.com/amrnt/8050103

Also I'v added a method to reset the enum column to its default value.

If you are interested I will open a pull request.