WIP creation of apps.embeds

Starting this PR so everyone can view progress. The models are in essentially final form. The tests for the models and the default backend are essentially complete.

Note that *testing may not work* because I've been trying out my refactor of Armstrong.Dev (so Tox works great ;)). I haven't committed my `tox.ini` or `fabfile.py` on purpose.

We have been using the groundwork for this code in production since February.

## TODO
- [x] Admin
- [x] South migration
- [x] Embedly Backend
- ~~YouTube Backend~~ (to be a new feature request)
- [x] tests for above
- [x] Readme documentation
- [x] Embedly requires a `settings.py` value and I haven't considered the error handling if that value isn't there
- ~~whether or not to include additional helpers that dovetail with what will be new Apps.Images~~ (possible separate PR after Apps.Images is done)
- [x] templates!
- [x] method for telling Arm_Layout where to find templates based on embed type (e.g. photos render differently than links)
- [ ] new version of ArmLayout with the [new backend](https://github.com/armstrong/armstrong.core.arm_layout/pull/8)
- [ ] support files for Arm.Dev
- [x] new version of [embedly-python](https://github.com/embedly/embedly-python/)
- ~~a possible Admin "preview" feature~~ (to be a new feature request)