deprecate Socket.socket_type

Socket.type has worked for ages
update constants from libzmq-4.1-dev
add versionadded to docstrings
add zmq.auth to API docs
update changelog in preparation for 14.1
update zmq-dev message
remove Python 2.5 hacks from setup.py

it's been long enough
Merge pull request #492 from minrk/changelog-14.1

preparations for releasing 14.1