PROTON-371: Created the Qpid::Proton::Link class.
PROTON-371: Created the Qpid::Proton::Sender class.
PROTON-371: Created the Qpid::Proton::Receiver class.
PROTON-371: Created the Qpid::Proton::Session class

Also created the helper source module with functions for working with
instances of Qpid::Proton::Session.