Add class implementing MessagePack serialization

MessagePack is a space-efficient binary data interchange format. I am going to
use it to encode profiling data in ProfilerSimpleUDP. The official PHP
implementation is provided as a C extension, so using it would further
encumber migration to HHVM. This patch adds MWMessagePack, a class implementing
a pack() method for encoding native PHP values as MessagePack byte strings. The
implementation is based on <https://github.com/onlinecity/msgpack-php>, but
revised for clarity and conformity with MediaWiki coding conventions.

Change-Id: Id2833c5a9da659cb13ec1330de9dd57138ada9c8