First version of an external ids mixin
Apply external ID support to the top-level record types
Stringify hash keys in indexed documents for the sake of consistency.
When applying relationships, throw an error if linked URIs don't exist.
Index external IDs and provide a controller for querying them.
Lock down the external_id endpoint for now.

This may change as permissions become more fine-grained.
Merge pull request #218 from hudmol/external-ids

External ids