Add callback support

This patchs provides the code to add callbacks per event or for each
events. For now the priority is not handled so the callbacks are
processed in a FIFO manner and global callbacks are processed before
per-event callbacks.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>