Add option to omit creating an instance reader method on class_attribute

[bensie]

In an effort to make class_attribute a little more backward compatible with the now-removed class_inheritable_accessor, I've added the ability to disable the instance reader method.

Here's an example case of where it's needed:
https://gist.github.com/1033533

TL;DR: You can't override the reader method if the module extends ActiveSupport::Concern because of the order that things get mixed-in.

We're struggling to get Carrierwave running on Rails master as we remove uses of class_inheritable_attribute -- if there's any way this can be merged into 3-0-stable and 3-1-stable, that would help a ton. Thanks.

[bensie]

Hold off on this for a bit -- should have tested all of Rails instead of just active_support. Currently breaks uses in ActionController.

[josevalim]

Could you please provide pull requests for 3-0-stable and 3-1-stable?

[bensie]

@josevalim Sorry for breaking the build and thanks for fixing. I will open working pull requests for 3-0-stable and 3-1-stable.