Problem: zbeacon needs to be actorized

Solution: park existing zbeacon API in zbeacon_v2, and rewrite the class
as an actor. Had to do a few other changes to make this work:

- zsock_option now includes an API for setting zactor sockets; I'm not
  yet sure the extra methods are necessary, or whether it'd be better
  to overload zsock_set_xxx and zsock_xxx and use 'void *' for the
  socket argument (as done in zmsg and zstr and such).
Problem: zsock isn't consistent on polymorphic sockets

Clarified documentation somewhat and removed zactor_ layer over
zsock_option. All zsock_option methods now accept a zsock_t *,
zactor_t *, or a libzmq void *.
Merge pull request #635 from hintjens/master

Problem: zbeacon needs to be actorized