signal messageReceived moved to base class

New Qt5 Signals & Slots ( see :
http://qt-project.org/doc/qt-5.0/qtcore/signalsandslots.html ) need this
signal only in the base class.

This way next example can be done :
nzmqt::ZMQContext* context = nzmqt::createDefaultContext( this );
nzmqt::ZMQSocket* input = context->createSocket(
nzmqt::ZMQSocket::TYP_SUB, this );
input->subscribeTo( "topic" );
QObject::connect( input, &nzmqt::ZMQSocket::messageReceived, &myObject,
&myObject::myMagic );
//or even lambda functions can be used
QObject::connect( input, &nzmqt::ZMQSocket::messageReceived, [=](){
doMagic(); } );