[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

Add ZMTP connection heartbeats

This commit adds ZMTP connection heartbeats described in
http://rfc.zeromq.org/spec:37/ZMTP.

Typo fix

Problem: zmq::socket_base_t::connect fails on tcp ipv6 address

Rebase3

The shared reference count was not shared but copied. msg_t cannot
store the refcnt itsef but has to store a pointer to an externally
allocated (shared) refcnter. The changes to lmsg are reverted to
use content_t again. Howver, this introduces an allocation in v2_decoder
when creating the message which can be avoided. When allocating the reception
buffer, space is allocated for the maximum number of reference counts
(8192 / max_vsm_size = 8192/64 = 128 zmq:atomic_counter objects). This
increases the buffer by 128*sizeof(atomic_counter) = 128*4 = 512 bytes only.
When creating a message, the refcnt member is set to the address of one of the
pre-allocated atomic_counter_t objects. To do so, a new msg_t type zcmsg
is introduced because msg::copy must discriminate between the message types
when releasing memory.