Added support for MsgPack encoded responses (www.msgpack.org)

Hi there, since i am implementing MsgPack (www.msgpack.org) binary encoding for the responses of one of my REST API's, i decided to update CocoaRest Client which is the tool i use everyday for API debugging so that it supports MsgPack.

Please take a look at the quick mods i implemented, it would be great to have this into the master branch!

Cheers, Diego.