[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

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