Tracking changes to zproto class properties
Problem: lack visibility of current set of peers

Solution: add zyre_peers method which returns a zhash of the current
peers. The caller can use the hash keys (peer identities). There is
no other useful information in the hash table.

Fixes #246
Merge pull request #247 from hintjens/master

Added zyre_peers method
platform header missing from msvc directory, due to gitignore rule
Merge pull request #248 from twhittock/master

platform header missing from msvc directory, due to gitignore rule