Fix ClassCastException in tryClimb building inheritance map

The interface uses the generic Collection type but the implementation
an ArrayList. Fix by creating an ArrayList from the Collection. Resolves
this crash observed when updating MCPC+'s plugin loader:

https://gist.github.com/agaricusb/5471423
Fix null superclass in inheritance map

Object has no superclass, so don't add it to the list.
Also allow InheritanceMap setParents null as a shorthand for none.