Initial models, fields and default backends.
Initial tests
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.
South migration
Admin for the Backend model
Backends - calling without a URL returns None
Embedly - move invalid response logging into call()
Embedly - catch server/network errors and wrap them in our InvalidResponseError class
Tests - refactor and a few new tests
Embed Model - rework the way `response` is handled.

- can now set an invalid response, which makes a ton more sense. Nothing will save and the other attributes won't set anyway.
- ignore attempts to set the same response.
- get_response()
- update_response() can handle None
Simplify the `url` and `backend ` fields. All they do now is clear the response if changed. It's the Model's job to know if a Backend should be assigned not the individual fields.

Main change - the Backend is auto-assigned on save() now not when the field changes. This in itself is simpler. We also have a `choose_backend()` that can be used at others times.
Minor change - `backend` can be blank. This will help in the form/Admin use.
Test fix - Django 1.5 doesn't like this use of Fudge for some reason. Rework the test a different way.
South is optional so don't rely on the import
Refactor so I can share the code with a test method.
(super spiffy) Embed Admin with Preview ability. Also tests.
Fail more obviously when the required EmbedlyBackend API key is not in settings.