add the required rel attribute for Siren sub-entities
maintain interface compatibility across all adapters
trigger build via empty commit
allow providing an array of values for a siren sub-entity rel.

For Hal and Json Api, if the entity name is an array, we just grab the first
name in the array.
merge in upstream
Merge pull request #45 from apsoto/add_siren_subentity_rel

add the required rel attribute for Siren sub-entities