Update msgpack to 0.5.8.

2013-12-23 version 0.5.8:

  * Move to the new github repository msgpack/msgpack-c
  * Support the new deserialization specification
  * fixes the problem of unpack helpers for array and map with 32bit compilers (#37, #38)
  * Other bug fixes and refactoring: #46, #41, #36, #35, #33, #32, #30, #29, #28, #27, #26, #25, #8, #3
  * Update of documents: #23, #18, #17

(obache)