Fixed #398
Deprecated zmsg_push by zmsg_prepend

- method was unsafe, taking ownership of the frame (and destroying it)
  without nullfying the reference.
- deprecated zmsg_push (and also zmsg_wrap which has same issue)
- added new zmsg_prepend () using safe API style

Cause of this change was 2-3 hours lost in zproto tracking down obscure
bug caused by use of zframe_push(). Once is enough. :)
Merge pull request #399 from hintjens/master

Fixed #398