ActiveRecord mixin

cc: @joeyAghion

The README was lacking documentation on how to include `Garner::Cache::Binding` into ActiveRecord classes and  instances. (Thanks @skwp for catching this!) Assuming that we'll soon add some of the "extra" support to ActiveRecord that we currently provide for Mongoid (e.g., "virtual bindings" via `Model.identify(id)` and class-level bindings), I'm adding a fairly spartan `ActiveRecord` mixin.