-Rewrote instance_hook_perform_block to take an instance_hook_token_t and use that token to fetch the original hook; more reliable and less error-prone than the last approach
-Fixed another retain crash on ARC when calling dealloc IMP
-Reversed order of hook destruction on dealloc; now destroys the hook before calling super (apparently the reverse causes strange issues on ARC)
Updated readme to reflect changes in instance_hook_perform_block API