implements batch sending

rather than writing a single message per trip through the underlying
transport, writes are batched and sent as a group. This drastically
improves the system call / application code ratio and tcp packet
utilization rates when sending lots of small messages
use libc++ for non-c++11 branches on mac os x
Merge branch 'master' into batched_send
update changelog
Merge pull request #343 from zaphoyd/batched_send

Batched send
force the offset to be calculated first to avoid confusing MSVC's checked iterators references #342

Thank you tmoers for reporting
Merge branch 'master' into core_dependencies

Conflicts:
	changelog.md