avoid 'During handling of the above exception…' on failed import

use Python 2-compatibly syntax for `raise Exception from context`
Merge pull request #548 from minrk/raise-from

avoid 'During handling of the above exception…' on failed import
generate darwin/platform.hpp on 10.6

removes TCP_KEEPCNT and TCP_KEEPINTVL from bundled libzmq, which are new on 10.8 or 10.9.
use pushstream.send in test_auth

avoids hang if POLLOUT is never set on self.server (4.1.0)
workaround bug in 4.1.0

zeromq/libzmq#1052 makes it impossible to cleanup contexts
Merge pull request #550 from minrk/4.1.0

use pushstream.send in test_auth
support ssh multiplexer in tunnels
Merge pull request #551 from mattja/ssh-with-mux

support ssh multiplexer in tunnels