Add generic method support

[cwebber]

Instead of using .m.foo() method stuff, it would be better and cleaner probably to support generic methods.

[cwebber]

Instead of attaching methods to objects at all, there would be a toplevel function that you could extend with new type definitions. This works like so:

# toplevel method
dbm_save = GenericMethod("Save object to the DBM")
# definition for the Note type
@extend_generic(dbm_save, Note)
def note_dbm_save(note, db):
   bla bla blaa # Note-specific code goes here