Rails 4 support.

I've made changes necessary to use proposal in Rails 4.

This includes changing the dependency to `rails ~> 4.0`, so it will currently
refuse to run in Rails 3.x.  It could be relaxed, but testing across Rails
versions would be required. I'd lean towards requiring Rails 4, and users of
Rails 3 can stick to the older version, as per the updated README.

* 82995a7 Rails dependency changed to ~> 4.0, was ~> 3.2
* c3d70a2 Regenerated Dummy Rails app using Rails 4.
* 107e4c7 Removed attr_accessible. Rails 4 uses strong_parameters.
* 510ff9d ActiveRecord scopes as lambdas.
* f710b95 README updated re. Ruby and Rails versions.