Added z85 codec to ZMQ API

* Removed redundant Z85 code and include files from project
* Simplified use of headers in test cases (now they all just use testutil.hpp)
* Export zmq_z85_encode() and zmq_z85_decode() in API
* Added man pages for these two functions
Merge pull request #660 from hintjens/master

Added z85 codec to ZMQ API