Problem: specification of short nonces is unclear

Solution: tighten requirement of implementation to check that peer is
using a strictly-incrementing short nonce. Also, remove statement that
CurveZMQ does not use CurveCP-style incrementing short nonces (it does
and must, to prevent replay attacks).
Problem: need right to block/ban troublesome users
Merge pull request #58 from hintjens/master

Problem: CURVE mechanism does not check short term nonces