Redis config loader

[nevans]

Based on #144 (to not duplicate autoload/require efforts) and #142 (because this loader ought to always be wrapped in a throttling or memoized loader.

Allows basic setting/loading of configs in redis. By default it stores a separate config per app/host/env, but that can be easily changed (by choosing names that are shared among pools).

[nevans]

n.b. either solve #135 or clearly document that user will need to reconnect in after_prefork