Solution: add manpage
* Added new man page for this * Added test case, in tests/test_security_curve.cpp * Noted in zmq_utils.h that these methods are documented
* 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