Merge pull request #1251 from bradleypriest/rootFromType

Add test for custom typeFromRoot
Merge pull request #1244 from bradleypriest/keyFor

JSONSerializer recognises keyForAttribute/keyForBelongsTo if defined
Basic DS.ActiveModelSerializer implementation
custom JSON root
Get extraction working
rename rootForType hook to match
- Duplicate RESTSerializer serialize methods
- Basic polymorphic serialisation support
Add polymorphic hooks to JSONSerializer
Change polymorphic serialisation to a single hook
Use a less obtuse test object
update test