Basic DS.ActiveModelSerializer implementation

Not ready to merge. But food for thought on an ActiveModelSerializersSerializer :smile: See #1180 

- [x] Underscored URLs - `pathForType`
- [x] Normalize attributes - `keyForAttribute`
- [x] Serialize attributes - `serializeAttribute`
- [x] Normalize root keys - `typeForRoot`
- [x] Serialize root keys - `serializeIntoHash`
- [x] Normalize belongsTo keys - `keyForRelationship`
- [x] Serialize belongsTo keys - `serializeBelongsTo`
- [x] Normalize HasMany keys - person_ids -> people - `keyForRelationship`
- [x] Serialize HasMany keys - people -> person_ids - `serializeHasMany`
- [ ] Normalize polymorphic objects
- [x] Serialize polymorphic