Cocoa: Client-side support for Multiplexing Services on any Transport and Protocol

This pull request brings client-side support for multiplexed services as specified at [THRIFT-563](https://issues.apache.org/jira/browse/THRIFT-563) and also some additions ac4ef15 to the Thrift Cocoa tutorial 1e5c6d6 from https://github.com/apache/thrift/pull/46 relating to the usage with multiplexed services. This patch is tested with the Erlang server implementation from https://github.com/apache/thrift/pull/42