Calls to POST API have include=tags

closes #2998, closes #3131

- update PostSerializer to use DS.EmbeddedRecordsMixin
- create PostAdapter to include include=tags in query params for POST and PUT
- set include=tags for various GET post requests
- change PostModel to have { embedded: always } instead of { async: true }
- update Ember-Data to beta8 from beta7
- fix the way isDirty works with updated tags
- put model.updateTags() into model.save() to make sure it always happens