Refactor Repository and Veritas::Adapter objects

* Collapse Repository::{InMemory, Persistent} into
  Repository only, which accepts a name + adapter
* Support registering veritas adapters by their
  URI scheme and implement the following dedicated
  adapters:
    Veritas::Adapter::Postgres
    Veritas::Adapter::InMemory
* Remove Environment#setup in favor of passing a
  repository config hash to Environment#initialize
  (typically used via Environment.coerce)
* Refactor spec_helpers to be self contained for
  a given spec run (unit/integration)