add Role::Throwable

	new file:   lib/POEx/ZMQ/Role/Throwable.pm
ffi backend Error objs & ErrorChecking role

	new file:   lib/POEx/ZMQ/FFI/Error.pm
	new file:   lib/POEx/ZMQ/FFI/Role/ErrorChecking.pm
add constants (and constants generator)

	new file:   lib/POEx/ZMQ/Constants.pm
	new file:   tools/zmq_constants.pl
cleanups

	modified:   lib/POEx/ZMQ/FFI/Context.pm
	deleted:    lib/POEx/ZMQ/Role/Throwable.pm