expanded usage of handleFindQuery
Support pushing embedded json into fixture adapter

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