* class_only was a keyword arg of get() and all() that was mistakenly
passed on to Plugins. Be sure to strip it from the keyword args
before instantiating Plugins. (Reworked API probably should either
not instantiate Plugins or take the args for the Plugin as a separate
list and a dict.)
* Checking required base_classes was only done in get() and only if
class_only was False (ie: that Plugin was instantiated). This meant
that different plugins could be found depending on whether the call
was to .get() or to all() and whether it was for classes or instances.
Fixed so that required base_classes are always checked.
7ce130212f