ClassLoader silently fails with old-style class object

[mathturtle]

When an object that is not the class object of a new-style class is given to the ClassLoader, it fails silently and returns an empty set of plugins. The fact that the subclasses parameter must be a new style class should be mentioned in the documentation and at least a warning message should be printed when the loader does nothing (and it should probably be an exception raised rather than a warning message when the parameter is of the wrong type).