callback aggregator for AnyEvent watchers

 The module allows You to create callback's hierarchy. Also the module
 groups error and result callbacks into one object.
 .
 Also the module checks if one callback was called by watcher or not.
 If a watcher doesn't call result or error callback, error callback will
 be called automatically.
 .
 Also the module checks if a callback was called reentrant. In the case
 the module will complain (using "carp" in Carp).
 .
 If a watcher touches error callback and if superior didn't define error
 callback, the module will call error callback upwards hierarchy.