Add info on sending underscored JSON for create/update operations

In 0.13 the REST Adapter would send underscored JSON for create/update
requests.  Beginning in Beta 1 the REST Adapter sends camelized JSON.

This PR updates TRANSISTION.md to show how to get back to underscores.
Add ActiveModelAdapter and Serializer
Document the AMS adapter
Move everything to a package
Update TRANSITION.md

Added some clarification around ActiveModelSerializer terminology.
Add metadataFor
Merge pull request #1286 from thomasboyt/metadataFor

Add metadataFor
Merge pull request #1206 from bradleypriest/ams

Basic DS.ActiveModelSerializer implementation
Add some basic docs
Override normalizeRelationships to extract polymorphic type correctly
Merge pull request #1287 from bradleypriest/ams

AMS basic docs and polymorphic extraction
Merge pull request #1285 from eccegordo/patch-2

Update TRANSITION.md
Add ActiveModelAdapter and Serializer
Document the AMS adapter
Move everything to a package
Update TRANSITION.md

Added some clarification around ActiveModelSerializer terminology.
Add metadataFor
Add some basic docs
Override normalizeRelationships to extract polymorphic type correctly
Fixes fetches in the loading state

Fixes #1281 Fixes #1280