Add async timeinterval

[petervaro]

Add support for simple time-out and repeat interval schedules. Each object can have these callbacks, and each can have exactly one:

kb_Error <object>_bind_on_time(<object> self,
                               double miliseconds,
                               kb_Error(*)(<object>, ...);
kb_Error <object>_bind_on_interval(<object> self,
                                   double miliseconds,
                                   kb_Error(*)(<object>, ...));
kb_Error <object>_unbind_on_(time|interval)(<object>);

As well as more advanced forking:

kb_Error <object>_on_exit(...) => should join all processes