Refactored EmbeddedRecordsMixin to push records instead of sideload

Previously EmbeddedRecordsMixin just moved records around and relied on sideloading for their extraction. Now they are pushed to the store as they are parsed.

Benefits include:
1. Less confusing code
2. Out of the box sideloading support
3. Support for using EmbeddedRecordsMixin with JSONSerializer
Can we keep the payload example's in the code for the api docs? I've had to link to test often to help people understand the payload for embedding as well as highlighting what Ember data expects in a payload.