remove inappropriate `const void`s from cffi backend

on zmq_device and zmq_proxy.

some compilers seem to balk at the discrepancy.

closes #454
minor cleanup in auth
fix default value for copy in send_string

should have been True
Merge pull request #456 from minrk/send_string

fix default value for copy in send_string
fix possible use of deallocated python object

 __dealloc__ method might be called when the object is partially deleted, so
e.g. its methods must not be called. Fix this by copying the code of the
methods which require no python object internals.

Closes gh-458.
Merge pull request #460 from juliantaylor/dealloc-fix

fix possible use of deallocated python object
remove Python references from dealloc methods

finish up 450
Merge pull request #455 from minrk/no-const

remove inappropriate `const void`s from cffi backend
Merge pull request #459 from minrk/3.4

remove Python references from dealloc methods
typo
Fix order of imports in zmq.green.eventloop.ioloop

Previously the import of zmq.green.Poller would be clobbered by
the version of Poller from zmq.eventloop.ioloop.  Switching the
order of the imports ensures that the green version is used.
Merge pull request #462 from rfk/master

Fix order of imports in zmq.green.eventloop.ioloop
minor test cleanup
test context options
support context options in CFFI backend
add pad byte to CFFI binary data alloc

ensures strings are null-terminated
use c_ prefix instead of low_level_
make zmq_msg_t opaque struct

don't need to know the size, shouldn't care when it changes
Test accessibility of custom IOLoop subclass via base IOLoop.instance.
Cleanup green implementation of ZMQIOLoop.instance() and friends.