Resolve equivalents on Person POST/PUT

Extract an IdentifiedModelTransformer super to
DescribedModelTransformer which specifies abstract resolveEquivalents
and resolveSameAs methods for handling the resolution of equivalent's
URIs and SameAses to LookupRefs.
Merge pull request #340 from atlasapi/post-people-equivs

Resolve equivalents on Person POST/PUT