Register a handler. A handler must be an instance of a subclass of Handler. All handlers
are organized in groups with a numeric value. The default group is 1. All groups will be
evaluated for handling an update, but only 0 or 1 handler per group will be used.
add_handler(handler, group = 1L)A Handler instance.
The group identifier, must be higher or equal to 1. Default is 1.
You can use the add (+) operator instead.
The priority/order of handlers is determined as follows:
Priority of the group (lower group number = higher priority)
The first handler in a group which should handle an update will be used. Other handlers from the group will not be used. The order in which handlers were added to the group defines the priority (the first handler added in a group has the highest priority).