Begin to scaffold core objects

Add Configuration, Instance, and Feature objects and start to scratch
out specs. Add skeletal "on" functionality.
Add percentage on and off

Spiking a little to try to understand the interface of Feature.
We'll fix it in post.
Scratch out Redis backend

Adds a redis backend for storage and change notification.
Add cohorts to toggle features

Cohorts allow a subset of identifiers to have a feature enabled. This
is checked before the percentage check so that a given cohort a users
will always see a feature if it is added.

* Add testing infrastructure (shared backend example group, helpers, etc)
* Add Feature#on? for checking if a given identifier should see a feature
* Break out responsibilities
WIP
Add experimental postgres backend