update APIs for libzmq-3

mostly internal, this updates to the official libzmq3 APIs.

Actual changes:

add Context.set/get for context options
Context.set/getattr will set context options or default socket options

todo:

- [x] expose Context.get/set
- [ ] expose Message.get/set
- [x] alias Socket.getsockopt/setsockopt Socket.get/set for consistency
- [ ] update out-of-date setup.py messages about zmq versions
- [ ] test on Windows
- [ ] test on old libzmq
- [ ] merge PR #294 (this depends on that)