Stop using ActiveSupport::Concern

ActiveSupport::Concern does not deal with some changes to Rails 4.1
autoloading, leading to issues with multiple included blocks being
defined when modifying files in the Rails development environment.

Not defining included as a block seems to allieviate this issue, so this
commit strips the use of ActiveSupport::Concern, and just deals with
.include and .prepend directly.

Closes #12.