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

tests/test_spec_rep.cpp

@@ -17,8 +17,6 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include <stdio.h>
-#include <stdlib.h>
 #include "testutil.hpp"
 
 const char *bind_address = 0;