Calls to POST API have include=tags

closes #2998
- 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
- make call to get tags from model in editor.edit route synchronous since the tags now exist in the store
- change casper test to wait for call to posts api with `?include=tags`