Response class - make field name attrs private
Response class - make `data` and `fresh` attrs private
Refactor tests into more universal shared code.
Add Embedly Backend with tests.

Notes:
- requires a fork of the official Embedly Python library because our model caches the full JSON response data from the API (in the `response_cache` JSONField). The official library uses a custom class (reimplementing a core language data structure) and cannot be serialized.
- Tests won't run because they require an `EMBEDLY_KEY` in settings. I'm writing this with new Armstrong.Dev code (https://github.com/armstrong/armstrong.dev/pull/12) and, since that is waiting for commentary, I'm not committing the settings code that works with the new scheme.
Use our own "armstrong.apps.embeds" named logger. This seems like a good idea and we can silence our logging during tests.