Forum: Ruby on Rails Is this a use case for an engine or a railtie and why/how?

I have the following files and would like to somehow package them 
together
in a gem however I'm unsure of what I need to create. An engine, a 
railtie,
or something completely different?

/lib/unit.rb
A unit class to handle a common requirement across my models

/app/concerns/unitify_concern.rb
A class method #unitify that I can call in my model which sets up my
callbacks, validations, scopes, attribute readers/writers and
attr_accessible.

/config/initializers/unitify.rb
Where I've added a few custom Ransack predicates.

I'm open to opinions - it almost seems like too small an amount of code 
to
be worth separating like this. However, I would really like to learn the
process of packing a gem for a rails app. I'd also like to get 
experience
testing in isolation and this seems like just the right size place to 
start.

Thanks,
 - FJM