Merge pull request #38 from ralphbean/master

Allow the user to set TCP keepalive options
replaced calls to setsockopt/getsockopt with set/get

Relates to txZMQ issue #39.

Fix bug introduced by api changes to pyzmq (core/socket.pyx).
Merge branch 'master' of git://github.com/smira/txZMQ
Double compat checking (ridiculous!)
Compatibility with both old and new pyzmq (13.0.0 and 2.2.0.1)