Support pushing embedded json into fixture adapter

[ksykulev]

Users can now push json with embedded records via store.pushPayload while using the fixture adapter.

It supports the following method signatures:

store.pushPayload('user', { id: '1', name: 'john'})
store.pushPayload({
  users: [{ id: '1', name: 'john', project: '1' }],
  projects: [{ id: '1', name: 'ember-data', users: ['1'] }]
})
store.pushPayload('user', {
  users: [{ id: '1', name: 'john', project: '1' }],
  projects: [{ id: '1', name: 'ember-data', users: ['1'] }]
})

Doesn't use serializerFor like the store for non-fixture adapters
So as a side effect:
typeForRoot not yet supported
normalize(Links|Payload|Hash) not yet supported