rename "load_handlers" to "load_classes"

Also make it load the exception class if a "default_exception" is
defined.

Keep "load_handlers" around as a deprecated alias for "load_classes."
document alternative "default_exception" usage
v1.1.0

  - Rename "load_handlers" to "load_classes" and make it also load the
    exception class if a "default_exception" is defined.

  - Make "load_handlers" a deprecated alias for "load_classes."

  - Minor documentation adjustments.