Removed arbitrary VuFind\Cart reference in MultiAuthTest to prevent confusion; added test involving bad service name.

Refactored Auth plugins to avoid dependence on generic ServiceLocatorAwareInterface; injections are now more targeted for clearer dependencies.  Also added some missing comments.

Refactored authentication interface for compatibility with plugin manager (moved configuration setting from constructor to setConfig() method).  Added some partial unit tests for uncovered authentication methods.

Added unit test for ILS authentication; made ILS authentication module more testable (with optional dependency injection) and less error-prone (isset checks to avoid notices).