Improved pickling of abstract base classes. Every abstract base class
contains two WeakSet attributes _abs_cache and _abc_negative_cache. These 
sets cache the result of subclass tests. It is advisable not to pickle the 
content of those caches, because it could contain unpickleable objects.
This commit changes sPickle to replace _abs_cache and _abc_negative_cache
by new empty WeakSet instances.
Bump version to 0.1.3.