Add support for per-plugin DB-backed settings

We need to be able to query the DB. Ideally we'd do this by depending on
a future "dradis-core" gem containing the core classes, but we don't
have that yet, so we default to a configuration setting:

Dradis::Plugins::configuration_class

By default this is 'Dradis::Core::Configuration' but can be easily
overriden in an initializer (mainly in Pro, where Configuration is not
namespaced to Dradis:: yet).
Require files below the module definition so we can use .setup in the Engine class