add constants from libzmq-3.3.0
updateconstants workaround for ZMQ_NULL
update constants
strip null-termination from bytes-sockopts

excluding IDENTITY
utf16 test fix
use numpy.isnan

to make sure no nan
update int sockopt test exclusions
rebuild constants.c when constants.pxi is updated
test libzmq security
Merge pull request #401 from minrk/newconstants

Add constants from libzmq-3.3.0

3.3.0 adds some security functionality. This adds the relevant constants, allowing libzmq security with pyzmq.