Whitespace
Rename Settings#all to Settings#all_settings

* In Rails 4+ there `where` method depends on ActiveRecord#all. Setler
  overrides the `all` method which causes issues with the where clause.
* The easiest fix is to break the Setler API why renaming this method.
Updates README re: api change
Make `attr_accessible` dependent on Rails version rather than respond_to
Add deprecation warning for #all in Rails 3
Merge pull request #25 from iamvery/rails4

WIP: Rails 4 compatibility