Add option to initialize MDP worker with a context

This can be useful, for instance, when you want to
build a Majordomo application in that the workers
are threads instead of independent processes
Update examples to the modified MDP worker interface
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 ()