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
Merge pull request #461 from minrk/pypy-ctx-set

support context options in CFFI

also a little cleanup in tests, and ensure zero pad on bytestrings,
to avoid buffer overrun with CFFI keys, since libzmq uses strlen.

closes #457