Update zmsg_pushstr () function calls

The new czmq API (2.1.0) changed the zmsg_pushstr ()
signature. Now, in order to push a variable-argument
string into a zeroMQ message it is necessary to use
zmsg_pushstrf ()
Update zsocket_bind () function calls

The new czmq API (2.2.0) changed the zsocket_bind ()
signature. This, together with commit e6b3a5af05
fix issue #48.
Merge pull request #51 from lerwys/master

Fixes to make MDP work with czmq API 2.2.0