Renamed Message to Frame
Changed Message to Frame in examples
Split Frame factory functions again

zmq_msg_init() is not necessarily the same as zmq_msg_init_size(0), and
the latter was inaccessible when the two functions were combined.
(In truth, zmq_msg_init() IS exactly equivalent to zmq_msg_init_size(),
apart from a single "if" statement – the size check –  but that is an
undocumented implementation detail which we shouldn't rely on.)
Renamed Frame.reinit() to rebuild()

I never liked reinit(), and rebuild() is the name used by the C++ API
(CPPZMQ).