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.

I'll be on vacation for the next 2 weeks so discuss away and the other TBC guys can chime in. We have been using the groundwork for this code in production since February.

## TODO
* Admin
* South migration
* templates!
* a templatetag helper
* utility method for telling Arm_Layout where to find templates
* Embedly Backend
* YouTube Backend
* tests for above
* readme additions for above
* 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
* a possible Admin "preview" feature