cffi backend implementation
Update msg send and recv functions definitions
Add zmq_ctx_new and refactor cffi tests to use

a TestCase instada of test functions
Add linger procedure to context.term()
Fix contexto __del__ and remove unnecessary 

instance method
import constants groups from zmq.sugar
remove socket.rcvmore attribute
use _ to describe private backend attributes
fix cffi zmq_poll implementation
remove cffi duplicated constants grous and import

from sugar in socket.get
remove cffi Poller and unused polling code
remove instance attribute from Context and

make specific cffi attributes private
add encoding info to cffi_core.constants
Change SkipIf to raise SkipTest in cffi tests

to achieve py26 compatibility
comestic import changes (use () notation)
remove unused cffi import on cffi test case
fix device/proxy functions arguments names to

match current API
remove unnecessary trim bytes function
only do initthreads for CPython
fix Context destruction / Socket gc