Expose RunLoop

[ben-crowhurst]

auto service = make_shared< restbed::Service >( );
auto session = make_shared< Session >( service->get_runloop( ) );
session->get_runloop( );

[ben-crowhurst]

It would be nice to perform the following:

Session::wait( ); //wait for dispatch queue == 0, complete.
Session::wait( callback ); //wait for dispatch queue == 0, call complete handler.