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.
Revert "Stop using ActiveSupport::Concern"

This reverts commit 841ab3e5066e31fee63c808d51be7f69853d8362.
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.