raise better ImportErrors

mainly raise the original ImportError when the preferred backend fails,
rather than the fallback, which isn't typically informative.
add zmq.utils.sixcerpt

excerpts of six.py. Currently, only `reraise`
use sixcerpt.reraise

for Python 2/3 compatibility
Correct parameters in on_recv_stream callback

As per http://zeromq.github.io/pyzmq/api/generated/zmq.eventloop.zmqstream.html#zmq.eventloop.zmqstream.ZMQStream.on_recv_stream
the signature is 

    callback(stream, msg)
Merge pull request #544 from sk2/patch-1

Correct parameters in on_recv_stream callback
Merge pull request #543 from minrk/better-error-on-failed-import

raise better ImportErrors
changelog for 14.3
14.3.0
typo in changelog rst links
14.4-dev
ignore compiler/config.json
Add Cython binding for zmq_msg_gets
Add tests for message properties in test_security
Return None instead of bytes() in Frame.gets