Break out ActiveRecord type-casting logic into `MR::TypeConverter`

This breaks out the ActiveRecord type-casting logic into a
separate utility. This is setup so that the factory module can
use it with it's data generation methods. The `MR::ReadModel`
fields will continue to use ActiveRecord's type-casting, they
will just do it through this new utility class.
Merge pull request #139 from redding/jcr-type-converter

Break out ActiveRecord type-casting logic into `MR::TypeConverter`