Adding `lib/activeresource.rb` to the project.

This allows to remove the `require: active_resource` from the Gemfile
and just use `gem 'activeresource'`.
Updating changelog.
Merge pull request #95 from lunks/add-activeresource-file

Adding `lib/activeresource.rb` to the project.
Merge branch 'master' into fix/errors-from_type/use-known-attributes

* master:
  Updating changelog.
  Adding `lib/activeresource.rb` to the project.