THREAD PRIORITY option documentation

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