Merge pull request #421 from minrk/changes14

start changes for 14.0
bundle libzmq-4.0.1
link libsodium if available in bundled libzmq
rename stopwatch to utils in cython backend

place to put everything from zmq_utils.h
expose zmq.curve_keypair

in Cython backend
add curve_keypair to cffi
curve_keypair docstrings
test curve_keypair