Merge pull request #45 from hintjens/master

Fixes to ZRE and ZMTP
Fixed rules for identities

Previously ZMTP required user-defined identities to NOT start
with binary zero. This was a cause of hard-to-find errors in
apps using randomized identities, e.g. UUIDs, and as the rule
solves no actual problem, we removed it, in libzmq and in the
protocol spec.