Added documentation about context's options ZMQ_THREAD_SCHED_POLICY and ZMQ_THREAD_PRIORITY

Allow explicitly setting buf sizes to 0

LIBZMQ-195 allow explicitly setting sndbuf and rcvbuf to 0 (see https://support.microsoft.com/en-us/kb/201213)

on Windows, handle WSAENOBUFS like WSAEWOULDBLOCK

add message type sanity check in msg_t::rm_refs

[decoder.hpp] Fix misspelled inclusion

[decoder*] Style fixes for consistency

Addressed comments from T. Köppe

- Replaced stdlib.h with cstdlib
- Made single-argument constructors explicit

"zero-copy" raw_decoder

A memcpy is eliminated when receiving data on a ZMQ_STREAM socket. Instead
of receiving into a static buffer and then copying the data into the
buffer malloced in msg_t::init_size, the raw_decoder allocates the memory
for together with the reference-counter and creates a msg_t object
on top of that memory. This saves the memcpy operation.

For small messages, data is still copied and the receive buffer is reused.

Clean-up: Rename internal macros consistently.

Add C++11 atomic support also to atomic_counter_t.

atomic_ptr.h: clean up the clean-up

Add missing relaxed memory ordering.

Add support for <atomic> in C++11

ignore unbind tests

update gitignore with additional tests

add tests for #949

Fix units and default values for heartbeats options

Set the ZMQ_HEARTBEAT_TIMEOUT to default to the value of
ZMQ_HEARTBEAT_IVL if it's not explicitly set.
Change the units of ZMQ_HEARTBEAT_TTL to milliseconds in the API
and round down to the nearest decisecond so that all the options
are using the same units.
Make the maximum heartbeat TTL match the spec (6553 seconds)

Add documentation for ZMTP heartbeats/fix test