Fixed some problems in encode method

* zframe_push is destructive, which isn't clear from the API (it should
  take a pointer to a pointer). So pushing the routing_id leaves it in
  an uncertain state.

* Changed to using new zmsg_prepend, which depends on CZMQ master post
  pull request #399.
Added server generator
Install zproto_server_c.gsl
Merge pull request #26 from hintjens/master

Fixed some problems in encode method