Add association management to `MR::Model` mixin

This adds support for association management using the
`MR::Model` mixin. This allows defininig associations that
interact with models, but drive their records associations
correctly. This allows the developer to not worry about getting
a record and building a model or pull a record from a model.
Merge pull request #4 from redding/jcr-associations

Add association management to `MR::Model` mixin