Create test Rails 4 app
Allow Rails 3.2.14 and greater as well as Rails 4
Create models and tests for Rails 4 app
Use strict mass assignment sanitizer in Rails 4 app and issue warning if this sanitizer is not specified

Also ensure that mass_assignment_authorizer is always available by running inside "included" block and check for "protected_attributes" gem.
Updates to README.md describing Rails 4 support