Changes to improve Rails 4 support

The following changes enable Rails 4 support, while also maintaining compatibility with Rails 3.2+

1. Call attr_accessible only when using ActiveRecord < 4 or when using the 'protected attributes' gem
2. Rename the longest database index to allow tests to run in Rails 4
3. Update the README regarding the use of filtered params
4. Change gemspec and require statements to remove references to protected_attributes, and allow ActiveRecord > 3.2