Allow setting `primary_key` as a read model field type

This updates the read model fields to allow `primary_key` as a
type. This type-casts the column the same as it does integers.
This enhances the factories when they providing data. Any read
model columns listed as primary keys will automatically get
unique integers.
Merge pull request #108 from redding/jcr-read-model-allow-primary-key

Allow setting `primary_key` as a read model field type