Add base `MR::Record` mixin

This adds a base `MR::Record` mixin that adds a `model` accessor.
This is used by the `MR::Model` mixin to set the model that is
using the record. This allows the record to access model logic
should it need it in callbacks or validations.

@kellyredding - This will need to be rebased with #3 and #4 to see the actual changes easily.