Rails 4 support

Hi guys,

I was trying to do some Rails 4 updates to our Rails app, and stuck with inherited_resources and it's dependencies.

So this pull request is the first in series to get IR working on Rails 4 (and Ruby 2.0 if necessary).

I didn't know what approach to take in terms of supporting previous versions of Rails such as Rails 3.1 and 3.2. I think the most straightforward way is to make master branch Rails 4 compatible (rather than introducing if/else conditions for different Rails versions) and keep Rails 3.x work on support branches (pretty much the same way as Rails does with master, 3-2-stable, 3-1-stable, etc.). So I removed support for Rails 3.x in this pull request.

Please let me know if you'd like to take other approach and/or have any questions, concerns, etc.