Implement msgpack support

This patch implements support for msgpack (http://msgpack.org).

MsgPack is a binary transport for schemaless data structures. It can be thought as a binary serialization of JSON, though this is not formally a correct definition. MsgPack is very popular in Japan and is increasingly used as a compact serialization / data storage format.

The present patch makes httpie understands the "application/x-msgpack" content type, and decode it appropriately to the standard output. It is very useful when working with an API that implements this serialization format.

Testsuite is OK after this patch is applied.