[Enh] Pier-ExternalLogin

A small framework to enable external validation plugins to be added and configured for Pier Login. I am using this with a plugin (not included yet) which validates the user/pass against a MYSQL query, then it logs in this user using an internally defined pier-user (or as I call it a "role") as a template. Finally I perform a second query to obtain a UserData record, and a potential invalidation is performed on "any other criteria", such as an account expiry date. You add your plugin by subclassing PUExternalValidationPlugin, and select the plugin to use in the application configuration. Finally your plugin gets the opportunity to customize the Login Dialog's descriptions. This enables you to customize the presentation of the dialog if you add Magritte-ComponentDecoration 's. You should also be able to add extra fields, though I leave this as an exercise for the reader. enjoy Keith